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:
and others or combinations of these.
(In the example image here, aliasing also shows within the imagined continuous source signal itself.)