For me canonical when used as an adjective often means derived from existing structure. Let's say I have an inner product space $S$ with some nice inner product ($\langle{}x,y\rangle{}$ for $x,y \in S$ ). Now let's say I need a norm on this space. I could pick an arbitrary one. There are many possible choices. I would consider the one induced by the inner product ( i.e. $\sqrt{\langle{}x,x\rangle{}}$ ) to be the canonical choice. Even if another was more useful for the problem at hand.

For me canonical often means derived from existing structure. Let's say I have an inner product space $S$ with some nice inner product ($\langle{}x,y\rangle{}$ for $x,y \in S$ ). Now let's say I need a norm on this space. I could pick an arbitrary one. There are many possible choices. I would consider the one induced by the inner product ( i.e. $\sqrt{\langle{}x,x\rangle{}}$ ) to be the canonical choice. Even if another was more useful for the problem at hand.