Words without simple synonyms

Many of such words would be quite technical, not "primitives" or least-common denominator; and would not be essential to a " basic" vocabulary. obviate, parallel, isosceles, gastroenterologist, pahoehoe, anemometer, sphygnomometer, pyroclastic, enantiomer.... These are not "irreducible", but it takes more than one word to " reduce" them. (of couse, now someone will come up with one-word synonyms just to refute my offhand examples).

I would suspect that invented languages would have a lot fewer synonyms than English, which has in many cases inherited or adapted multiple terms for the same thing from various languages.

Here's an article about Ithkuil, an invented language ostensibly much more efficient (more precise, yet more concise) than English. http://www.newyorker.com/magazine/2012/12/24/utopian-for-beginners

I've been looking for a list of words that don't have synonyms. I've had no luck. I have however researched the idea for about two years now and the closest I have gotten to previous work on the matter was by the linguist Morris Swadesh. He is a little dated on his work. Wikipedia has an appendix of "Swadesh lists" in various languages: http://en.wiktionary.org/wiki/Appendix:Swadesh_lists

These words tend to lack synonyms in their respective languages. Words like colors or body parts. There are verbs as well though.

There is the word "Univocal" but I'm not sure if this is what you mean.

OED

1.b. Of terms, etc.: Having only one proper meaning or signification; admitting or capable of a single interpretation or explanation; of which the meaning is unmistakable; unambiguous.Opposed to equivocal

adj. 2. Now esp. in Logic.

1843 J. S. Mill Syst. Logic I. i. ii. §8 A name is univocal, or applied univocally with respect to all things of which it can be predicated in the same sense

Most prepositions have no true, or even close, synonym.

However, given the nuances of English, there are very, very few "true synonyms". English words "carry baggage" (often depending on the language from which they came), e.g. king is not identical to monarch; quick is not the same as fast, etc. Also context plays an important part.