The French word used in Ballet to decribe a bend of the leg at the knee while keeping the back straight. It is derived from the past participle of the word "plier" meaning "to fold, bend."

It is pronounced as "plee-ay," with the emphasis on the first syllable, and is generally considerd the most common step in Classical and Romantic Ballet. This has given rise to the frequent lament among dancers, "Another day, another plie." (Note that in this phrase "day" and "plie" rhyme, hence the humor).

And in case you were wondering, all ballet steps are named in French.