It can be a little bit grey.. as in what’s ‘assured’ to you may be different to what’s ‘assured’ to me.
In a practical sense it usually comes down to an error/misjudgement and if something/someone saves your bacon and prevents it resulting in a loss of separation then it’ll usually go down as a loss of sep assurance.
Example:
2 aircraft converging at the same level, conflict alert activates and the controller turns one quickly enough that they don’t lose a radar standard then that would be deemed a loss of sep assurance; or, 2 aircraft crossing, one climbed through the level of the other because you don’t recognise the conflict - supervisor sees the whole thing including the climbing aircraft reaching 1000ft above with 5 seconds to spare but controller didn’t even realise.. that’s a LOSA.
Probably best summed up as ‘achieving separation not by fluke or by having your ass saved by the system tools and/or another controllers situational awareness’