A word is (i) a free morph, or (ii) a clitic, or (iii) a root or a compound possibly augmented by nonrequired affixes and augmented by required affixes if there are any.
The definition given here is from Haspelmath (2023). It does not define "word" as a "natural" concept, but it is largely in line with the way linguists use the term. The definition indicates that "word" can hardly be a basic concept of grammar, and if the morphology/syntax division is based on the "word" concept, it can hardly be a basic architectural division.