1. To treat, show, or tp consider something as equivalent to something else; such as, equating money with happiness.
2. To reduce something to the same level or value as something else.
3. To put something into the form of an equation involving an equality.
4. To be or appear to be the same.
5. To reduce to an average; that is, to make such corrections or allowances which will reduce something to a common standard of comparison.
6. To make something equal or equivalent.

In color vision, the physiological faculty of combining two colors to match a third, so as to combine red and green to make a homogeneous yellow.