what is a synonym?

The songwriter Burt Bacharach gave us a good definition of a synonym:

"A synonym is a word you use when you can't spell the word you first thought of."

Burt Bacharach - A synonym is a word you use when you...

Here are a couple of other definitions:

one of two or more words or expressions of the same language that have the same or nearly the same meaning in some or all senses

Synonym Definition & Meaning - Merriam-Webster

A synonym is a word that has the same meaning as another word (or nearly the same meaning). For example, beautiful and attractive are synonyms of each other because they both refer to someone or something that looks good.

Synonym: Definition and Examples | Grammarly

The problem is 'nearly'...

The Grammarly definition points to why it's a good thing to have synonyms in the sense that we need a better choice of words and we need to avoid using the same old vocabulary again and again (and again).

It then looks at the different types of synonym, starting with:

Absolute synonyms:

Grammarly says:

Absolute synonyms are words that mean exactly the same thing; there is no difference in meaning. You can use absolute synonyms interchangeably; one synonym can replace another without changing the message.

Examples: identical — indistinguishable... drink — beverage... insect — bug

The problem is that all of these examples have very different registers (ie, formality/informality)

Jay Doubleyou: register

Jay Doubleyou: high culture > popular culture --- high register > low register

So says this from a discussion forum:

The basic reason why absolute synonyms (or absolute anything, really) don't exist in natural languages is because whenever two forms have no differences whatsoever in register, meaning, or emotional value, someone is bound to exploit the unused form distinction so as to give it a register, meaning, and emotional value of their choice, and some of these choices will stick, producing non-absolute synonymy, or absolute non-synonymy -- though often only in certain speech communities.

english - Do absolute synonyms exist? - Linguistics Stack Exchange

There is no such thing as a synonym!

It's very handy to be able to use a thesaurus - or list of words with a similar meaning.

But:

It is only a fiction if one holds fast to the notion that synonyms must be exactly equivalent in their meaning, usage, and connotation. Of course, under this strict view, there will never be any “perfect” synonyms. No word does exactly the job of another...

Word for Word | Lapham’s Quarterly

You have to rely on more than just a list, though:

A thesaurus is a handy catalog of synonyms full of exciting (astonishing, flashy, lively!) words we can use in our writing. While it’s tempting to grab an electrifying word and go when we look for a synonym, this practice presents some pitfalls.
Every word in the English language has its own particular place, and it’s even possible to claim that there is no such thing as a true synonym.
Search for the word house in Thesaurus.com, and the options include not only home (which we all know has a cozier connotation than house) but also apartment, condo, mansion, and shack. Kennel and pigpen are there too. (Think of the trouble you’d get into if you chose one of these without knowing its definition … yikes!)
To select the best alternative to house, you have to rely on more than just this single listing in the thesaurus.

Is There Such A Thing As A True Synonym? - Dictionary.com

Finally, here's a good discussion from a Cambridge Dictionary forum:

There is no such thing as a true synonym in English. Discuss! - About Words - Cambridge Dictionary blog

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