Term for a person with absolutely zero knowledge of a topic

For a brand new beginning-beginner, I would use the term "novice".

The term virgin is often used

A person who is inexperienced in a given activity or field: "all these [factors] kept me a gun virgin well into midlife" (Michael DiLeo).

American Heritage

The term is often preceded by an adjectival noun describing the area of naiveté.

Supplement

As has been noted in some comments, virgin has a long history of a sexual connotation, and many would consider it informal or slang. Its first usage in a non-sexual context was probably considered metaphoric, but it seems to have become a fairly standard, albeit only appropriate for certain circumstances.

It also tends to be used to describe a person about to venture into a new arena, rather than someone who is stubbornly avoidant of initiation into the mysteries to be found in the love of a new range of experiences (and all the attendant Shades of Grey)!

In the context you mentioned in your OP. I would say:

Entry-level

There are other contexts as well.

greenhorns

In construction type jobs, people who are new to the profession

freshmen

In academia

rookies

In the police force

Other words worth mentioning are:

• naif
• green
• amateur
• inexperienced
• ignorant

A lot of answers are focusing on having no experience in a subject. But if you need a word describing having no knowledge in a subject, I'd go with ignorant.

I would suggest neophyte.

neophyte |ˈnēəˌfīt|
noun
a person who is new to a subject, skill, or belief: four-day cooking classes are offered to neophytes and experts.
• a new convert to a religion.
• a novice in a religious order, or a newly ordained priest.

Based on the phrase "green field" that appears in the question, I'd suggest "blank slate", which is defined by Merriam-Webster as "someone or something that is still in an original state and that has not yet been changed by people, experiences, etc.".

In the context you provide – a training course – I would go with this:

• Uninformed
• Beginner
• Intermediate
• Expert

These are students and teachers who will read this. Many of the terms provided in other answers (e.g., "ignorant", "virgin") may meet the requirement for the definition, but are inappropriate in terms of context.

"Uninformed", in this particular context, strikes me as non-judgmental.

"Newcomer" is also very good (but already provided by @Sawbones).

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Also, just fyi...

Many organizations actually use "Beginner" for the level where a person has no knowledge at all about the topic.

Then comes "Basic" or "Novice".

So it's common to see this:

• Beginner
• Novice (or Basic)
• Intermediate
• Expert (or Advanced)

You can find plenty of examples online. In my quick review, the one thing they all had in common was "Beginner" came first (and was synonymous with "uninformed", "newcomer", "entry level", etc).

You could use an abecedarian:

: one learning the rudiments of something (as the alphabet)
_{Merriam-Webster}

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Since there is some curiosity about the origins of the word: The word is indeed a play on ABCD, but the word is quite old:

abecedary (n.)
"primer, alphabet table," mid-15c., from Medieval Latin abecedarium "an ABC book," neuter of adjective abecedarius, used as a noun, from the first four letters of the Latin alphabet. Abecedarian (adj.) is attested from 1660s.
_Etymonline

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Just noted, that the etymology definition offers a good alternative as well: primer. While this is not a good term for the person, it is a good term for the category of the course.

The uninitiated is a good non-judgemental term.

Use the word "novice." Neophyte is popular here but it is used for people new to a belief or religious order. Based on your context no one would say they are intermediate with a religion. But they would say that for a skill, in the same way they would say "I'm a novice."

"Newbie". You could even use slang and shorten it to the new, hip term "noob"

A slew of people said "novice" at about the same time - that would probably be my first choice for a safe but well understood term.

But, "initiate" (also with a religious or cultural undertone) would be well enough understood by most people.

From another age, probably too light hearted for business use but OK for eg a conference course description is tyro which actually suits your meaning very well.

If a touch of humor is acceptable, then a genuine term, which many people may not have met, but whose meaning will be clear - both due to its position on the list and it's obvious enough derivation is abecedarian Agh! - I now see that that has been suggested - it's still a good word - just not as original as I'd thought.

"entry level" does not seem to have been suggested - a term that is in very common use in exactly this context.

• Entry level
• Beginner
• Intermediate
• Expert

Ignoramus would fit nicely at the top of your list

Oxford:

An ignorant or stupid person.

I'm a complete ignoramus when it comes to Artinian rings of Krull dimension zero.

In the context of learning, admitting total ignorance is not a bad thing, it's an indicator of where to start study.

While I like neophyte, I also offer "nescient" as an option as it describes a complete lack of knowledge or total ignorance. Sort of where I put someone who has developed an interest in a subject, but not yet acquired any real knowledge and having to work they way up to "beginner".

The phrase rank beginner is sometimes used to describe someone who has no experience whatsoever (to distinguish from someone who has a little experience but is still at the "beginner" level).

the term Beginner is defined by CED as: a person just starting to learn a skill or take part in an activity.

1. a person or thing that begins.
2. a person who has begun a course of instruction or is learning the fundamentals:
   swimming for beginners.
3. a person who is inexperienced; novice.
   _{Dictionary.com}

If you place any of the so far suggested synonyms before beginner you probably risk confusing the learner.

In horse riding a beginner can be any one of three levels: 1. Total Beginner; 2. Advanced Beginner; and 3. Confident Beginner. Followed by Novice, Intermediate, Advanced, and Professional. See: Different Levels of Riding Ability

The OP could adopt the expression Total Beginner, a level which clearly suggests someone who has no experience or previous knowledge of the subject.

• Total Beginner
• Advanced Beginner
• Intermediate
• Expert

A useful word I haven't seen mentioned yet is layman, someone who has no or little knowledge of a particular subject.

One might say "in layman's terms" before explaining something technical to someone with no specialist knowledge of that area.

Going by the title of your question, I suggest absolute beginner - I've seen it used in various educational contexts (such as the title of this book, tutorials etc).

I would say that a person who does not have any knowledge of a topic is...

unfamiliar

: not frequently seen, heard, or experienced

...with the topic.

While not explicitly a noun, I think the word expresses a distinct distance (in my opinion) from the subject than the next given rank of "Beginner" which (again in my opinion) expresses already at least some partial understanding and closeness to the subject.

• {what goes here?} Intro
• Beginner
• Intermediate
• Expert

Or you might want to change some of the other level names. For example:

• Beginner
• Intermediate
• Advanced
• Expert

There is a also a possible level above Expert: Challenge