Does spending movement to stand up from prone trigger the secondary damage effect of the booming blade spell from the Sword Coast Adventurer's Guide (p. 142)? The spell says:
If the target willingly moves before the duration expires, it immediately takes 1d8 thunder damage, and the spell ends.
I don't see any specific rules definitions for "movement." Based on definitions in other editions, my opinion is that a character "moves" when they walk or use some other "special type of movement" such as jumping, flight, swimming, etc: when they move from their original position, rather than just standing up, dismounting, or otherwise reconfiguring their posture but remaining in the same place.
The spell doesn't say "if the target willingly takes any action." Therefore, attacks and environment interactions are probably fine. The spell doesn't force you to remain perfectly stationary like a statue. Given this, it seems clear to me that doing things that use movement but don't change your tactical position don't count.
Some places in PHB190-191 use the phrase "deduct... from your speed" instead of "spending [X] feet of movement." This might be a clearer way of looking at it. Some actions, such as standing up, reduce the amount you can move in the turn even though they are not "movement" for the purposes of triggering spell behavior.
For a useful corollary, it makes sense that mounted movement would still trigger the effect, even though you're not using your own movement or deducting from your speed.
Although it doesn't affect the lack of definition in 5e's rules as written, D&D 4e says "Whenever a creature, an object, or an effect leaves a square to enter another, it is moving." 4e rules don't apply to 5e, but they provide insight into the background of D&D rule design.
According to the implications of the Jeremy Crawford ruling below, standing up from prone does not trigger booming blade. This is because the trigger requires the target creature to move, but even though standing up costs the target movement it does not actually move the target anywhere.
Question: For Booming Blade, did you intend for standing up from prone to trigger the extra damage? It costs movement.
Crawford's Answer: Standing up costs movement but moves you nowhere. To move while prone, you crawl or use magic (PH, 191).
(See: Twitter reference.)
In the DnD Basic rules for 5e it says that when prone you may spend half your movement to stand up (page 70), it then goes on to state that to move while prone your character must "crawl" in which case every foot of your movement costs an additional foot (page 71). since standing up costs half your movement and is therefor not crawling (and crawling is the only way to move while prone) standing up is not moving.
hopefully this helps
Booming blade now reads:
If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends.
It is now abundantly clear that standing up from prone does not trigger the secondary damage, since standing up from prone is not moving 5 feet.
