There is no consistent convention on what constitutes a missile vs. what constitutes a torpedo in Sci Fi, from what I've seen.
IRL, a missile is generally used to describe a guided munition that flies through the air (unguided munitions being just "rockets"). Torpedoes, on the other hand, are munitions that are propelled though the water, and may or may not be guided.
The thing is, there is no difference in mediums when it comes to space (ie, no "water" or "air"). Therefore, any propelled and guided munition could be considered either a Torpedo, a Missile, or both! From what I've seen, it really comes down to what kind of feel a particular writer or game creator wants to evoke in their setting; "torpedo" has a stronger connection to historical naval warfare, while "missile" has a more technological advanced feel. Dropfleet makes interesting use of both terms; Missiles are short-range weapons that are fired en masse, while torpedoes are giant single-shot ship killer weapons.