This is the term used for the first
coil in a
transformer. It can be thought of as the
input. You pump
alternating current into the
primary coil, it
induces current in the
secondary coil (often through the use of a common core, but this doesn't have to be the case, such a "coreless" coil is often said to be "air core").
Of course this is a term of function. It is a just a coil, as is the secondary. If you turn a step-up transformer around, you have a step-down transofrmer.