I have a theorem of the following form
Definition. An object $y$ has property $X$ if... [2 lines...]
Lemma. If $y$ has property $X$, then ...
I could also simply write
Lemma. If $y$ has the property that [insert definition of $X$], then
But the definition of $X$ is long enough that writing it this way would be clumsy.
On the other hand, the definition of $X$ is only used in this lemma, so it should not draw too much attention, so what would be a good-writing-style of making the definition of $X$ take less attention while still using it to keep the lemma short? ($X$ has an intuitive english-language term).
I though of putting the definition in the proof of the Lemma but this comes accross as non-rigorous.