Extrusion (Modeling) is a term in 3D modeling where a 2D outline is given height, or some times moved along a path to become a 3D form.
Good examples are where text is turned into a basic 3D form, or when a single face on a mesh is made to extrude from or intrude into an object (usually automatically, by duplicating the face, defining a new surface for each set of duplicate lines, and moving the duplicate surface a distance on a line perpendicular to its orientation).
The term also sometimes applied (possibly incorrectly, but nevertheless is) to cross section modeling, where a series of 2D shapes are morphed in a sequence to form a 3D shape. This process often but not always involves curved paths that control both the degree of progression as well as making the line the morph follows bend instead of following a straight line.