What is the definition of "declare a move"?

Is it the same as performing a move (just gaining movement points)?

Or do I declare a move whenever I have the intent to enter a space?

Or do I declare a move when I want to enter a space that is e.g. 4 spaces away and only at the beginning of my movement not for every single space?

I am no native speaker so please bear with my questions.

So can Overwatch trigger when Loku has no LOS to my figure when I "start moving"?

As far as the current interpretation it means when the move (or attack) action is declared. (I don't think it applies to when a move is performed from an ability that is not an action.)

The wording would be different if it would be intended to trigger when spending movement points.

Overwatch can be used even if Loku does not have line of sight, but then the attack would abort during step 1. of the attack (Declare target). (Edit: there may be additional abilities that allow to ignore figures when determining line of sight.)

The only instance that would not be the case would be if Loku purchased the upgrade to target figures with recon tokens, ignoring other figures when determining LoS.

Yes, Combat Spotter would allow him (among other friendly figures) to ignore figures when the target has a recon token, but in the generic case it does not guarantee line of sight, although makes it much more probable.

Combat Spotter won us our game against Vader. I sacrificed Loku to mark him and then we spent all of our actions just pummeling him to death haha.

Cruelty toward Sith Lord, tsk tsk.

Combat Spotter won us our game against Vader. I sacrificed Loku to mark him and then we spent all of our actions just pummeling him to death haha.

Sacrificed Loku by him becoming only wounded, right? (If he is withdrawn, there goes Combat Spotter.)

Yes wounded was the result. Left only the medic droid with 2 HP for a win.