As children, we learn what a tree is when someone points to a tree and says, "tree". In this way, "what a tree is" is communicable.

No. This means that the label "tree" is communicable, and that with any luck, native ability to separate natural kinds will fill in what the label is meant to contain. The definition of "tree" is also communicable, but it doesn't involve pointing and isn't closely wedded to the internal mental understanding of treeness. (My mental understanding of treeness is something like "plant that holds its leaves or needles a ways off the ground with a large stick of wood". The definition involves exciting things like "apical dominance".)