In lighting, what is a kicker and when should I use it? (And is it any different from hair light?)
I'd usually use the term kicker to refer to any highly off axis light which strikes a glancing blow on the subject. This creates a highlight or gradient over a small area of the image in order to accent the area, define shape.
I [over]use this type of light mostly in portraiture, where primarily it helps define the unlit side of the face. It also adds drama, gives the image a modern look, here's a comparison with and without the kicker:
It can be more subtle than this, as in the following image. I find a tiny bit of light on the off side almost always improves a shot:
The kicker can have the effect of fattening the face a little so there are times when the standard short lighting pattern is preferable. Here's a not so subtle kicker using a bare strobe to the left:
The term rim light is also used for kickers though I find it's used more for the occasions where the light is so oblique that all you get is a strong highlight, as in the following image:
Jill Greenberg uses a pair of kickers in pretty much every shot
When you say kicker, you assume that the light is placed at an angle behind the person, on the other side of the subject from where your key light is. It is usually placed out of the frame.
A hair light can be a kicker, but it can also encompass the entire head, be placed directly behind your subject.