Definition of substitution. 1  1a : the act, process, or result of substituting one thing for anotherb : replacement of one mathematical entity by another of equal value. 2  2 : one that is substituted for another.
Homogenized milk is any sort of milk that has been mechanically treated to ensure that it has a smooth, even consistency. The homogenization process typically involves high temperatures, agitation, and filtration, all aimed at breaking down milk's naturally occurring fat molecules.
One of my tricks with homogenized milk is to start with a milk that has a low fat percentage, like a 1% or 2%, and then add the best commercial cream I can find to bring the fat content up to 3-4%. This has always set better for me than using regular whole milk.