Aliasing refers to perceptual distortions or artifacts which are caused by sampling errors when converting a continuous source signal, as seen by the eye, the camera or as imagined, into discrete values on a fixed digital raster.

Antialiasing (AA) is an umbrella term for all techniques which aim to lessen the unwanted perceptual artifacts, e.g. by:

1. lowering contrast
2. averaging or inventing new sample values
3. increasing resolution

and others or combinations of these.

(In the example image here, aliasing also shows within the imagined continuous source signal itself.)

further reading:

• Aliasing
• Jaggies