The term "double" has multiple meanings depending on its context.
In mathematics, it can refer to a number that is two times another number (e.g., 2x3 = 6). In sports, particularly baseball, it refers to a player's batting average when they have hit the ball twice as many times as they have struck out (i.e., having an equal number of hits and strikeouts).
In addition, "double" can also refer to two separate things that are similar or identical in some way (e.g., two cars with the same make and model, or two people who look alike). It can also mean a drink consisting of two measures of liquor served together with a mixer such as soda water or ginger ale.
Please specify which meaning you would like me to expand on further.