A definition can be as short as it needs to be to convey the complete, correct meaning the searcher is after, with nothing padded on top. The measure is completeness of meaning, not a target length. A definition satisfies the query the moment a reader walks away understanding the term correctly and fully enough for their purpose, and any words beyond that point are filler that slow the answer without improving it. The floor is meaning delivered in full, and the ceiling is the point where you have done that and should stop.
This means length follows the term, not a rule. A straightforward concept may be fully captured in a sentence or two, because that is all the meaning there is to convey, and stretching it would only dilute a clean answer. A term with conditions, exceptions, or a context the reader needs to grasp the meaning correctly will take more, because completeness genuinely requires it. The right length is whatever it takes to leave no real gap in understanding, which varies by how much meaning the term actually carries.
Treat any specific length you have in mind as a working guide rather than a standard, and worth re-checking against the actual term in front of you. The mistake is fixating on a word count in either direction: padding a simple definition to look substantial, or clipping a complex one to look tight. Both miss the point, because the searcher is not measuring your words, they are checking whether they now understand the term. Completeness of meaning is what they are testing for, so that is what you optimize against.
When you write a definition, ask whether a reader would now understand the term correctly and fully for the purpose that brought them, then cut anything that does not serve that understanding. Make it as short as full meaning allows and no shorter, with no padding to hit a length. Let the meaning set the length, and the definition will satisfy the query at whatever size that turns out to be.