Question

what is callus(in biology)?

Answer 1

You may use this term while studying tissue culture. It is an unspecialized , unorganized, growing and dividing mass of cells. It produced when explants (totipotent cells from plants like apical meristem) are cultured on the appropriate solid medium, with both an auxin and a cytokinin in a correct conditions. 

Answer 2

Plant callus (plural calluses or calli) is a growing mass of unorganized plant parenchyma cells. In living plants, calluscells are those cells that cover a plant wound.