, a dominant chord, with altered "pretty notes", those pretty notes being the ninth, eleventh, and thirteenth degrees of the chord
that prettify the basic dominant. Traditionally the dominant
(the chord built on the fifth degree of the major or minor scale) resolves to the tonic1
(the chord built on the first scale degree, which is the key the tune is in); altering those pretty notes, by raising or lowering them a half-step, takes you out of the key of the music, and provides a little extra oomph
(a technical term) when resolving to the tonic
An example: a G9 (G-B-D-F-A) chord resolving to C Major 7 (C-E-G-B). All the notes involved come from the C Major scale, and it can be a vanilla sound when used all the time (e.g. by some bad lounge pianist). In altered-dominant form, that G9 can be changed to a G7#5#9, in which the fifth and ninth degrees of the chord are raised a half-step, making it G-B-D#-F-A# (the sharp-five is the same note as the flattened thirteenth degree). A spicier sound, and it lends itself to melodies that also incorporate notes from outside of the key of the music (see: chromaticism).
1 More generally, a dominant is any chord built on a major triad, with a real or implied minor seventh degree, that resolves to a chord whose root is a fifth below the dominant's.