"Projection" is something of a catchall term for any ability that allows a mutant to manifest her presence somewhere she isn't. In general, this consists of the ability to manipulate the physical world via a force field or a duplicate that occupies a defined space. Certain psychics with power over illusion can simulate projection, but such illusions are by definition not "pure" projection, as they do not affect the physical world. Canon Invisible Woman's force fields would be considered projections, while canon Jean Grey's would not -- Jean repels objects with her telekinesis rather than projecting a solid barrier as Invisible woman does. Luke is a good example of an OC projectionist.