Khaki is a term used to describe both a type of fabric as well as a color. Khaki fabric is constructed out of cotton or linen which has been woven in a twill fashion. Typically khaki cloth is identified more readily by the color of the cloth rather than its construction.
The term khaki, when used as a color, comes from the yellowish brown color of khaki cloth. This color comes in many different shades and can range from a very light nearly white color to a very dark almost muddy brown.
Khaki cloth gained the majority of its popularity when it was adopted by many different militaries for the production of uniforms. In the United States khaki uniforms can be dated all the way back to 1898 when it was used during the Spanish-American war.
In modern times khaki is used more as a slang word to refer to a style of pants that are constructed out of khaki colored cloth. These pants are typically worn when a slightly dressy business casual look is desired.