How does Canada define it?

Under Canadian immigration law, permanent residence is defined by an objective physical presence test. According to Canadian law, as long as you are physically present in Canada for a cumulative period of not less than two years (or 730 days) in any five year period, you are maintaining permanent residence in Canada. So, for example, in any five year period, A Canadian resident could choose to live three years straight in the United States provided that he or she then returned to live in Canada for the last two years. Or, put another way, a Canadian resident could live a few months in the United States, then travel up to Canada for a few weeks then return to the U.S. for a few more months, then up to Canada for a few weeks etc., so long as the combined total of all those weeks of physical presence in Canada totaled two years. It is the resident's duty to prove actual physical presence through visa stamps in the passport, airline tickets and the like.