The term altered chords refers to the sharpening or flattening of degrees of a chord, usually the 5th or the 9th. This opens up many possible combinations and once you know the formula for any extended chord you can experiment with alterations to the degrees of the chord. The most common altered chords are dominants, so you may see a symbol such as E7#9. This refers to an E7 chord with a sharpened 9th degree added to it.