Good question. I think there is a lot of variation, depending on different points of view. I'm guessing people are meaning the last lover one was seriously caring about.
In my case, we were together 23 years, a mix of excellent and not quite so excellent; 20 years since the breakup, and we're in a long term good friends stage. I don't mind that he loves his next wife and she seems not to mind that he calls me a few times a year. I've been considering using his real first name on a2k on the few times I mention him, instead of the ax-like word, ex. I did that once, and it felt comfortable.