Altered scale
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In music, an altered scale is a scale in which all of the notes of the scale except the tonic have been flattened (lowered in pitch) by an interval of a half step from a major scale. For example, while the C major scale consists of the notes
C D E F G A B C
the C altered scale consists of the notes
C D-flat E-flat F-Flat G-flat A-flat B-flat C 
Another way to create an altered scale is to sharpen (raise in pitch) the tonic of a major scale by a half step; for example, when we sharpen the tonic of the B major scale, which has the notes
B C-sharp D-sharp E F-sharp G-sharp A-sharp B 
we get the C altered scale
C C-sharp D-sharp E F-sharp G-sharp A-sharp C 
the notes of which are enharmonic (identical, in the equal temperament system) with the notes of the C altered scale as it was first described on this page.
Another way to look at the altered scale is that it is identical to the seventh musical mode of the ascending melodic minor scale.
One may also think about it more abstractly, relative to the diminished scale or the whole tones scale. The altered scale starts with the first five pitches of a half-whole diminished scale and terminates in whole steps (hence called the "diminished whole tone scale). In jazz harmony it often functions as a more intriguing dominant chord as there is tension between the tonic and the fifth of the dominant chord (minor 6 away).
