Is "learnable" a valid English word?

Question

In my thesis I am quoting a passage from a paper in which the author* used the word "learnable" in the sense of something that is easy to be learned.

I have searched for the word in both the Merriam-Webster and the Cambridge English online dictionaries, but didn't find it there. Nevertheless, this word seems to be used quite a lot in many scientific papers, it returns 775,000 results on Google and a lot of occurrences in Wikipedia.

So I ask: is this a valid word in English or, perhaps, just a neologism?

^{* he is from the University of South Florida and has a common American name - just in case somebody finds this relevant}

Answer 1

Learnable has been in use at least since the early 17th century.

By and large, when you find that an ordinary suffix has been attached to a word of the sort it is ordinarily attached to, and that the resulting sense is exactly that which such compounds ordinarily bear, it’s a legitimate word regardless of whether it appears in a dictionary. That’s what linguists mean when they say that a suffix or idiomatic pattern is ‘productive’: that it is regularly used to create valid new words or phrases.

(And observe, please, that calling a term a neologism does not ‘invalidate’ it, whatever that might mean. To say a word is ‘valid’ can only mean that it is used and understood within its particular context or register, regardless of its age.)

In this case, -able is regularly attachable to any transitive VERB to express the meaning capable of being VERBed. Learn is a transitive verb, and learnable means capable of being learned. That’s all it takes.

Answer 2

Of course learnable is a word. Perhaps most people tend to use the term teachable instead of learnable; they do not seem to understand that teachable refers to someone who can be taught, and not something that can teach us anything. It's surprising how poorly Americans use the English language.