A family name that is generally passed down from generation to generation (via males of the family in Western culture). Women, traditionally (but not quite so much these days) often adopt the surname of the man they marry.

In modern culture it's fine for a male to adopt the females surname (though not that common), for couples to hyphenate their existing surnames into a new surname eg. if mary jones marries john smith they may join their surnames to be jones-smith or... possibly, it's fine to just create a whole new name (also not that common). Most commonly it is fine for each person to keep their own surnames and then fight when the children come along as to whose surname the children will get.

The last name in a pair of names, used mainly for people's names.

your last name