A function with the property that multiplying all arguments by a constant
changes the value of the function by a monotonic function of that constant:
F(kV)=g(k)F(V), where F(·) is the homogeneous function, V is a vector of
arguments, k>0 is any constant, and g(·) is some strictly increasing positive function.
Source: Deardorff's Glossary of International Economics
If g(k) = kX then F(·) is said to be homogeneous of degree X.
f(x,y) = 3x2 + xy + 2y2 is homogeneous of degree 2.
f(x,y,z) = x3 + 3x2y + 3xy2 + y3
+ 9xyz is homogeneous of degree 3.