Can a single word be an idiom or an expression?

Question

Can a single word be considered an idiom or an expression? If so, how does one tell the difference? For instance, words like, "dope!", "lit!", "sick!". These words mean completely different things than their original meaning when used in a certain way/context, "sick" means ill but in a certain context can mean "excellent". There are also words like, "cathouse", which have nothing to do with cats.

Are these words idioms or expressions? Confusingly, MacMillan says an idiom is an expression.

The website, Difference Between.net, says

Phrases can be taken literally while idioms are not literal in their meaning but figurative. Idioms can also be called colloquial metaphors. Another significant difference is that a phrase is always a group of words but a single word can on occasions be an idiom. For example: moonlighting, a single word meaning something different in the way it is used and it does not have anything to do with moonlight.

Visitors to UsingEnglish.com are invited to express their opinions on its language polls. In this instance, the question was Can a single word be an idiom? The results are shown in the graphic below.
70.99 % of voters replied “Yes".

A clear definition that shows the distinction between idiom and expression would also be very much appreciated.

Answer 1

Can a single word be considered an idiom or an expression?

From Lexico:

Expression

3. /count noun/ A word or phrase, especially an idiomatic one, used to convey an idea. ‘we have an expression, ‘You don't get owt for nowt.’’

So an expression is a commonly used set and fixed word or phrase to illustrate a concept.

Ex. of phrases

'To make a long story short'
'Your guess is as good as mine'
'As rich as Croesus'

Ex. of single words

'Great'!
'What'?
'Yuck'

These are common expressions which are understandable and literal, even when broken down into parts. In fact, most any often repeated word or phrase can be an expression. It is a very general grouping.

On the other hand, if an expression cannot be understood on its own in a literal sense, it becomes an idiom...

Lexico says that ...

Idiom

1. (is) A group of words established by usage as having a meaning not deducible from those of the individual words (e.g. over the moon, see the light).

Examples of idiomatic expressions

'over his head'
'to steal someone's thunder'
'Don't put all your eggs in one basket'

These are constituents, cannot be broken down, and are usually non-literal.

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MacMillan says mostly the same:

Expression

(countable) a word or phrase

Idiom

(countable ​linguistics) an expression whose meaning is different from the meaning of the individual words. For example, ‘to have your feet on the ground’ is an idiom meaning ‘to be sensible’.

[All emphasis mine]

...and ditto Merriam-Webster .

This would seem to indicate that 'idiom' can be seen as a subset of 'expression', but they are not necessarily the same thing.

So strictly speaking, I would say that interjections like...

"dope!", "lit!", "sick!"

...are expressions with idiomatic meanings, but they are not idioms, and in fact single words cannot be idioms, as John Lawler points out in comment.

Single words can't be idioms, but they can occur (perhaps only) in idiomatic expressions. The term "expression" seems to refer (not technically) to any frequently-repeated string of words, whatever its source or structure.

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Evidently, there is a linguistics term that might help bridge the gap between these words...

Multi-word expressions

...are linguistic objects formed by two or more words that behave like a ‘unit’ by displaying formal and/or functional idiosyncratic properties with respect to free word combinations. They include an extremely varied set of items (from idioms to collocations, from formulae to sayings)

Oxford Research Encyclopedia