F107 answered the question just fine, but it's early morning, I have nothing better to do, and I'm anal.
From AudioVideo101's Dictionary
"Directly in front of a speaker; position at a right angle (90-degree angle) to the front of a speaker enclosure on which the speaker drivers are located (the baffle). On-axis simply means taking a position directly in front of a speaker similar to sitting directly in front of a television. In this position, sound waves from the speaker come directly to you in a straight line."
"Position to the side of a speaker at some angle other than 90 degrees to the front of the speaker cabinet and its speaker drivers resulting in audio signals reaching the listener as a result of the speaker’s dispersion characteristics and often resulting in decreased output levels (as well as a different frequency response) compared to a position on-axis or directly in front of the speaker."
Actually, I like F107's explanation better, and it certainly was more succinct. Both addressed the horizontal axis (side to side), but F107 included the vertical axis (up and down), while AudioVideo101 didn't. However, I wanted to introduce you to AudioVideo101's dictionary. It's a good source to have when you run across an unfamiliar term.