What Is the biggest title in tennis?

A "grand slam" can mean two different things in tennis. Most people consider a "grand slam title" winning one of the four majors (Australian Open, French Open, Wimbledon, and the US Open) However, it can also mean winning all four of these events. If this is done during one season, it is most often referred to as a Calendar Slam (but can also be called a Grand Slam) and if a player wins all four of these events over the course of their career, it is accordingly called a Career Slam. Both are very rare.