Nope, it is where MC is taken from.
Netorare literally means "To be taken from." So any context where MC is stolen from basically.
here is the way f95 puts it:
View attachment 2428745
As you can see, neither require it to be cheating. (common misconception is that it means cheating, but an LI being raped or being oblivious also count, as MC is still being taken from).
A popular genre of NTR is childhood friend, which well.. given childhood friend is clearly not cheating. (still cucked tho :| )
Also purposeful or not, MC still gets cucked because of that.
edit: an easy way to think of it is anyone doing something with an LI without MCs permission/approval/knowledge. (that goes for male and female perpetrators btw)