A backlit cel is a type of special effects cel. The typical way to figure out if your cel is backlit, is, when you look at the back of your cel the paint on the back has been covered with another layer of black acrylic paint. This was done so that when they shine a light from the back of the cel causing a bright effect when filming, i.e. power ups and attacks are typical examples of this, there are no points of light coming through that break up the character image.
A backlit cel of Asuka from episode 9 of Neon Genesis Evangelion. Note that the white areas are paper leftovers from the douga being torn off.