to lay open by extending; to open wide; to spread out; to diffuse; as, a flower expands its leaves.-to cause the particles or parts of to spread themselves or stand apart, thus increasing bulk without addition of substance; to make to occupy more space; to dilate; to distend; to extend every way; to enlarge; -- opposed to contract; as, to expand the chest; heat expands all bodies; to expand the sphere of benevolence.-to state in enlarged form; to develop; as, to expand an equation. see expansion, 5.-to become widely opened, spread apart, dilated, distended, or enlarged; as, flowers expand in the spring; metals expand by heat; the heart expands with joy.