Why is Canada's leader called prime minister?

Answer 1

Canada has a parliamentary system where the leader of the party (perhaps allied with other parties) that has the largest number of seats in Parliament becomes the leader of the government. He or she is then the first (prime) minister among those who hold cabinet positions. If his or her party loses its majority in the parliament a new election can be held and if another party then gets enough votes the government changes. So it's very different from the US system where a person "presides" over the government for a set term.

Some parliamentary governments do have a president too, but that position is only honorary--he or she is above politics and stays on just to cut ribbons at bridge openings and the like and represents the country as a point of stability. In England (and some other European countries) the king or queen fulfills this rule--sort of like a living flag. In Canada the Queen's representative, the Governor-General, stands in for her.