# What's energy method?

When searching about PDE, I sometimes see the word "energy method". I want to know this method but there is no wikipedia page about it.

Suppose we have a parabolic PDE, says. Basically we exploit the structure of the PDE to create an energy-like functional $$E(t)$$ that is non-increasing over time, giving a priori estimate for important quantities (like $$L^2$$ norm) of the solution $$u$$ of our PDE. This can be used to prove uniqueness and other properties of $$u$$.