Similar to the notion of referring to an adhesive medical strip as a Band-Aid or facial tissue as Kleenex, when most people say "light bulb", they actually mean lamp. It is a matter of the correct technical term not being used in the common parlance. Likewise, when most people say "lamp", what they actually mean is light fixture or instrument. Most electrical engineers will tell a layman that the bulb is only the translucent casing (usually some form of glass) which holds in the gases used in the incandescent or fluorescent process.
As the joke goes...
Layman: How many electricians does it take to screw in a light bulb?
Electrician: It's called a lamp!