Elongation is the noun for becoming longer, or for making something longer than it was. A rubber band has elongation when you pull it; a shadow has elongation late in the day; a plant stem has elongation when it grows. Scientists and doctors like the word because it names the change itself — how much longer, and the stretching that got you there — not just the new size. In everyday talk people more often say stretching, lengthening, or getting longer; elongation sounds a little technical, as if it belongs in a lab, a clinic, or an engineering report. Though actually, it is not a fancy extra meaning: it is still just 'the going-longer'. People sometimes stretch it to any kind of drawing-out, even time, but careful speakers keep it for physical length. (Astronomy uses it in a different, specialist way: how far a planet appears from the Sun in the sky. That is not the sense a learner meets first.) The verb is elongate; elongation is the name of the process or the result.