What is the definition of a word? [closed]

A word is a string of sounds or letters which can be identified as a grammatical unit, larger than a morpheme, but shorter than a phrase, and playing a structural role in a sentence.

Word is not a term that has a single definition.

For one thing, what constitutes a word is different from language to language -- in polysynthetic languages, for instance, there's small difference between word and sentence.

For another, there are three common ways we use word
(the terms below are developed in P.H. Matthews' Morphology, Cambridge U Press):

1. We say the verbs lie and lay are different words, though they have some forms in common.
   This is the definition of word as Lexeme.

2. We say that the present tense form of lie is the word lies, pronounced /layz/.
   This is the definition of word as Word-Form, i.e, some particular form of a lexeme.

3. We say that a word pronounced /layz/ can have many meanings.
   This is the definition of word as Phonological Word, i.e, its pronunciation.

Here's an illustration of how it works in English.

So, there is no single definition; it depends on how one is using the term.

You're quite correct, though, that speakers make up words on the spot all the time,
and if they work and fill some need for others, they may be invented and used again.
But mostly they just die; sort of like mosquito eggs.