www.alphadictionary.com

I want to know that x% of ordinary English speakers have a specific word in their vocabulary. For example, what is the currency of the words matrix and segue? I think a speaker or a writer should use a vocabulary that challenges but does not confuse the listener or reader. How do I know the difference between a two-bit word and a six-bit word and when to use one or the other? Where do I go to find out?

In all my speech and writing classes I was encouraged to increase my vocabulary and then told not to use it. Strunk amd White pefer shorter words of germanic descent rather than longer and latinate words. Part of that, I think, is aesthetic, skewed to a more vigorous sound. Faulkner seemed to do ok while ignoring it. I once found a website, now lost, where you could copy and paste a paragraph and the site would return the grade level. Most journalism seems directed at 6th to 8th grade levels. As to your specific question, I have no idea how to discover how many know cat or feline.

The Collins Cobuild English Dictionary "helping learners with real English" ("real" is underlined in the original, but I can't do that here) provides "Frequency bands" with its entries.

Five bullet words are "common grammar words such as 'the,' 'and,' 'of' . . . . approximately 700 words in this band."
Four bullet words are "words such as 'argue,' 'bridge,' 'danger' . . . . approximately 1200 words in this band."

These two bands "account for approximately 75% of all English usage."

Three bullet words are "words such as 'agressive,' 'medicine' and 'tactic' . . . . approximately 1500 words in this band."

Two bullet words are "words such as 'accuracy,' 'duration,'
'miserable' . . . . approximately 3200 words in this band."

One bullet words are "words such as 'abundant,' 'crossroads,'
'fearless' . . . . approximately 8100 words in this band."

"Words in the five frequency bands . . . make up 95% of all spoken and written English."

The Collins Cobuild Dictionary is based on a corpus of 200 million word from a wide range of styles and sources most of it dating from 1990 or later. Sources are mainly British, but include American, Australian and Singaporian English

Of course, this information is the inverse of your question: It doesn't tell what percent of the population know a particular word, but I think anything in the top bands would be known to most speakers.

The Collins Cobuild English Dictionary "helping learners with real English" ("real" is underlined in the original, but I can't do that here)

Why can't you do that here?

Slava, 'cause us erudite folks ain't 'lowed to underline words.

Philip, I've been meaning to tell you that it's glaring bright ourside, but the sun will go down and we can sleep.

Yeah Perry, but mine has an almost Biblical aura that yours lacks. See Psalms 30:5b.