When you lengthen or extend something, you elongate it. A yoga teacher might encourage you to reach up as high as you can, so that you elongate your spine.
The verb elongate means "to make long or longer," and it stems from the Late Latin elongare, "to extend or prolong." When you stretch something out, especially when it's longer than it is wide, you can say you elongate it. This word can also apply to non-tangible things: Your principal might elongate the semester if you have too many snow days, and you might elongate your vowels when speaking a foreign language. In science, it's also used as an adjective, for long, narrow creatives like elongate fish (otherwise known as eels).