The direction of a sailboat with respect to the wind. (Not to be confused with point of sale.) In the following diagram, imagine the boat to be at the center of a circle, with the wind coming from 0 degrees. (In other words, if the boat is pointing into the wind, we'll say its direction is 0 degrees; if it's pointing directly away, its direction is 180 degrees. And yes, it's symmetric, so don't ask about degrees between 180 and 360.)

              \   in    /   
    close      \ stays /
    reach       \     /
                 \   /
   beam           \ /
   reach   -----------------
        broad      |
        reach      |

We can define the points of sail as:

< 45 degrees
in stays, or in irons. Unsailable.
45 degrees
close hauled or beating. The most you can point into the wind. Usually somewhere between 35 and 45 degrees in modern sailboats but may be slightly more or less.
45-90 degrees
close reach.
90 degrees
beam reach. The fastest point of sail.
90-180 degrees
broad reach.
180 degrees
dead run, or running before the wind. This is where you pull out the spinnaker or run wing on wing using a whisker pole.

All numbers are approximate, of course. If you're sailing at 89 degrees, it's appropriate to say you're on a beam reach.