an edging that is added to some woven fabric as finishing during the manufacturing process to prevent it from being unraveled, often made from different threads than the fabric itself and meant to be cut off before the fabric is used;
e.g. You should cut the selvage off the fabric before measuring it for use.
(noun)
broadly, the edge of something, the peripheral part, for example a portion of rock that has been altered at the edge of a mass of rock, for example to form volcanic glass;
e.g. The volcanic glass was collected from the selvage of the rock face.