Nestor George Chylak Jr. was an American umpire in Major League Baseball who worked in the American League from 1954 to 1978. He umpired in three ALCS, serving as crew chief in 1969 and 1973. He also called five World Series, serving as the crew chief in 1971 and 1977. He also worked in six All-Star Games: 1957, 1960, 1964, 1973 and 1978, working home plate in the second 1960 game and in 1973.

In baseball, the umpire is the person charged with officiating the game, including beginning and ending the game, enforcing the rules of the game and the grounds, making judgment calls on plays, and handling the disciplinary actions. The term is often shortened to the colloquial form ump. They are also sometimes addressed as blue at lower levels due to the common color of the uniform worn by umpires. In professional baseball, the term blue is seldom used by players or managers, who instead call the umpire by name. Although games were often officiated by a sole umpire in the formative years of the sport, since the turn of the 20th century, officiating has been commonly divided among several umpires, who form the umpiring crew. The position is analogous to that of a referee in many other sports.