1. A polynomial of the second degree which is homogeneous of the first degree in each of two sets of variables; thus, it is a sum of terms of the form a_ijx_iy_j , where x₁, . . . , x_m and y₁, . . . , y_n are two sets of variables and the a_ij are constants.

2. More generally, a mapping ƒ (x, y ) from E × F into R, where R is a commutative ring and E × F is the cartesian product of two modules E and F over R, such that for each x in E the function which takes y into ƒ (x, y ) is linear, and for each y in F the function which takes x into ƒ (x, y ) is linear.