I will consider a loss of territory the most obvious things, such as losing Ceuta, Melilla or Catalonia, but also a loss of territorial waters will be considered a loss of territory.
1,000@BrunoParga Gibraltar is not Spain as of today, the only way in which Gibraltar could affect this bet is by overtaking some land or water from Spain, btw they have done that in the past:
https://elordenmundial.com/mapas-y-graficos/expansion-gibraltar/
In yellow: Expansion between 1854-1907
In green: Expansion between 1908-2020
@LuisV Gibraltar is totally Spain from the Spanish point of view, no? They don't recognize it as British.
@BrunoParga I just checked in: https://www.lamoncloa.gob.es/brexit/gibraltar/Paginas/index.aspx
And it seems that for Spain Gibraltar is "a British colony in Spanish land", for the sake of this bet I'm considering that Gibraltar is "de facto" not Spain, so the only way for Gibraltar to affect this bet is by taking more land or water from Spain as I said before.
If you think I should review it in any other way please tell me.
@LuisV that seems pretty good! One thing I think it'd be good to have clear is whether you're including other types of sea areas, like the exclusive economic zone, or if you literally only mean the territorial waters. I think the latter fits best with the question, but the former might also apply (although I don't know if Spain has any disputes in that regard.)
@BrunoParga When I wrote it I was thinking in the following definition from Wikipedia :
https://en.m.wikipedia.org/wiki/Territorial_waters
"Territorial waters are informally an area of water where a sovereign state has jurisdiction, including internal waters, the territorial sea, the contiguous zone, the exclusive economic zone, and potentially the extended continental shelf"
As far as I know the only current dispute is the one with Morocco and the Canary Islands: