There is a _sense in which_ one might say that you "can" define a word any way you want. That is: words don't have intrinsic ontologically-basic meanings. We can imagine an alternative world where people spoke a language that was _like_ the English of our world, except that they use the word "tree" to refer to members of the empirical entity-cluster that we call "dogs" and _vice versa_, and it's hard to think of a meaningful sense in which one convention is "right" and the other is "wrong".
But there's also an important _sense in which_ we want to say that you "can't" define a word any way you want. That is: some ways of using words work better for transmitting information from one place to another. It would be harder to explain your observations from a trip to the local park in a language that used the word "tree" to refer to members of _either_ of the empirical entity-clusters that we call "dogs" and "trees", because grouping together things that aren't relevantly similar like that makes it harder to describe differences between the wagging-animal-trees and the leafy-plant-trees.
There is a _sense in which_ one might say that you "can" define a word any way you want. That is: words don't have intrinsic ontologically-basic meanings. We can imagine an alternative world where people spoke a language that was _like_ the English of our world, except that they use the word "tree" to refer to members of the empirical entity-cluster that we call "dogs" and _vice versa_, and it's hard to think of a meaningful sense in which one convention is "right" and the other is "wrong".
But there's also an important _sense in which_ we want to say that you "can't" define a word any way you want. That is: some ways of using words work better for transmitting information from one place to another. It would be harder to explain your observations from a trip to the local park in a language that used the word "tree" to refer to members of _either_ of the empirical entity-clusters that we call "dogs" and "trees", because grouping together things that aren't relevantly similar like that makes it harder to describe differences between the wagging-animal-trees and the leafy-plant-trees.