I think I had exactly the same definitions, only the other way 'round. Perhaps influenced by Star Trek TOS's Mirror-episode, I always felt that 'alternate realities' are the more different from canon... of course 'alternate universes' are implying quite a huge difference too... Argh!

Funny that the perception of the terms differs but everybody seems to agree that a Slytherin!Ron would be the minor diversion from canon, while gunslinger!Snape/horse-thief!Harry hatesex would be the larger diversion ;).