Why did the letter “o” disappear in the word “pronunciation”?

The answer to this question is that the 'O' got squashed out of the root by "rhythmic clipping".

First of all, one might think that there has never been an 'O' in the word pronunciation. In fact, the truth is rather different! Before the advent of dictionaries and word-processing spell-checkers, this word was often spelled with an 'O' in the second syllable. In fact the Oxford English Dictionary (not the Oxford Dictionary Online), gives five historical alternative spellings for pronunciation with an 'O' in the second syllable. One of these, it says, is still current, though not considered "standard". Here they are. The digits before the word indicate the centuries that spelling was used in. The third one in is meant to be current (1500s to the present day), though nonstandard:

• 15 pronouncyacyon, 15–16 pronounciacion, 15– pronounciation (now nonstandard), 16 prononciation, 16 pronountiation.

The rest of this answer is dedicated to why the letter 'O' is missing from this word in the modern spelling.

Let's consider the word pronounce. When we say this word, it is with an /aʊ/ sound in the stressed syllable. One of the typical spellings for this sound in the orthography is the sequence 'OU'. We can see this spelling-sound correspondence in the following words:

• abound
• profound
• found
• south

This vowel, /aʊ/, is often called the MOUTH vowel by phoneticians and phonologists. This vowel has a special relationship with the STRUT vowel, /ʌ/. The STRUT vowel is the vowel found in the words up, flood, and buck. This vowel is often found in words with the same root as words with the MOUTH vowel. Here are some examples:

• abundance
• profundity
• refund
• southern

Sometimes this change of vowel in the base is reflected in the spelling. This usually means that the STRUT vowel is represented by the letter "U". This is the case with the example words abundance, profundity, refund above. Because of the vagaries of English spelling, it isn't represented in the word Southern.

We might think that this is some random relationship. However, the vowel /aʊ/ is a diphthong. This means that it belongs to a family of vowels which may be characterised as being "long".

There seems to be a systematic relationship between many long vowels in English and other specific short vowels. If you speak English, these qualities will seem to be logical. In fact, they aren't. In terms of the actual sound there is almost no phonetic relationship between the long vowels and their short vowel counterparts. For example, the vowels in the words weight and bad are not very similar. We represent these sounds by the symbols: /eɪ/ and /æ/ respectively. However, these vowels have a very close relationship in the language. For example, the following words have the /eɪ/ vowel:

• grateful
• sane
• inflame

The following words, on the other hand, even though they have the same root, usually have the vowel /æ/:

• gratitude
• insanity
• inflamatory

This change from long to short vowels usually happens when there are extra syllables added to the base or root of the word. The more syllables there are in a word the more short vowels and the less long vowels we are likely to see. [This is a result of rhythmic clipping. This is when the vowels in the base of a word get shorter when extra syllables are added.] The following long and short vowels have this same relationship:

• eɪ / æ
• i: / e
• aɪ / ɪ
• əʊ / ɒ
• aʊ / ʌ

These relationships can be seen in the pairs of words respectively:

• chaste / chastity
• penal / penitentiary
• wise / wisdom
• joke / jocular
• south / southern

Note that the spelling of words may reflect common ways of enunciating them, for example:

• pronounce / pronunciation

In this example, as described above, we see alternation between /aʊ/ and /ʌ/. This is reflected in the corresponding spellings of "ou" and "u" respectively. However, it should still be noted, that this is because the spelling must reflect some type of pronunciation in the original instance. It is still the case that there are also alternative pronunciations. Very importantly, pronunciation may be pronounced /prənʌnsieɪʃn/ or /prənaʊnsieɪʃn/. Both ways of saying the word are perfectly fine, but the spelling reflects the first, not the second.

This is not the case for all alternations. For example we cannot say:

• waɪzdəm [wise dom]

for:

• wɪzdəm [wiz dom]

The sorter /ɪ/ vowel must be used in the longer word.

The Original Poster's question

The 'O' in pronounce disappears in the word pronunciation because it reflects a common change in the pronunciation of the English word, when we add more syllables to the base. Usually, in this case, the STRUT vowel replaces the MOUTH vowel for many speakers. However, this does not mean that the pronunciation of the second vowel in the word will change for all speakers. The retention of the full /aʊ/ vowel is also quite frequent in standard English.

The reason that this happens is that certain long and short vowels have a special phonological relationship in English. The vowels /aʊ/ and /ʌ/, MOUTH and STRUT are two such vowels and their spelling is often reflected in English orthography by the letters ou or u. However, sometimes many speakers still use the MOUTH vowel for words with a u spelling. This is perfectly fine!

For the record, English spelling is not systematic or well organised.

Hope this is helpful!

References: "pronunciation, n." OED Online. Oxford University Press, December 2014. Web. 28 December 2014.

While this probably doesn't add significant new content to the answers already provided, it seems useful to reproduce here the comments from Merriam-Webster on this issue:

Both the noun and verb come ultimately from the Latin verb pronuntiare. But when the Latin verb was taken into Anglo-French and later entered Middle English, that second vowel was sometimes rendered as -u- and sometimes rendered as -ou-. This meant that, from about the 1500s onward, we have evidence of both pronounciation and pronunciation in the written record. Eventually the noun standardized to pronunciation, but because of influence from pronounce, we do occasionally see pronounciation in print, and we also have evidence of a corresponding pronunciation for pronounciation (pruh-nown-see-AY-shun). The spelling pronounciation and the pronunciation that goes with that spelling are not considered a part of standard English. Using them could result in criticism or questioning.

We usually append the suffix -tion to verbs in order to make them nouns.
So pronounce (verb) + tion -> pronounciation.

But why is it written without an 'O' in the second syllable (even though the verb it derived from does have an 'O')?

Trisyllabic Laxing is the reason 'pronounciation' lost its O and became pronunciation. It's a process whereby a tense vowel is shortened if two (or more) syllables follow.

Examples:

• Profane, profanity --- /prəˈfeɪn/ -> /prəˈfæn.ə.ti/
• Impede, impediment --- /ɪmˈpiːd/ -> /ɪmˈped.ɪ.mənt/
• Divine, divinity --- /dɪˈvaɪn/ -> /dɪˈvɪn.ə.ti/
• Profound, profundity --- /prəˈfaʊnd/ -> /prəˈfʌn.də.ti/
• Pronounce, pronunciation --- /prəˈnaʊns/ -> /prəˌnʌn.siˈeɪ.ʃən/

Trisyllabic Laxing caused the pronunciation of 'pronounciation' to change which influenced the spelling.

---

OR

Pronounce is stressed on the second syllable, but when we append the suffix -tion, the primary stress moves to the penult. Words that end with -tion are usually stressed on the penultimate syllable.

The /aʊ/ diphthong (as in the word 'pronounce') seems to have a systematic relationship with the STRUT vowel /ʌ/ (as explained in Araucaria's answer and also explained in Trisyllabic Laxing)

When we add syllables, the vowels get laxed (shortened) and the /aʊ/ is likely to change to the /ʌ/ vowel.
Therefore, when we add the suffix -tion to the word 'pronounce', the primary stress moves to the penult and we get the /ʌ/ vowel in the second syllable of pronunciation.

Why does the O get removed from the second syllable of 'pronounciation'?

It's probably because we have the /ʌ/ vowel in the second syllable and the digraph <ou> in Modern English does not often represent the /ʌ/ vowel (there are a couple of exceptions though). So we remove the 'O' to match the spelling and pronunciation.