A series of pitches which move down by semitones is a descending chromatic scale. If this line is in the bass you could call it a descending chromatic bassline.
If the harmony above this bassline remains unchanged, the resulting chords can be described as a series of inversions. In popular music terminology this is most easily achieved using "slash-chords". For instance, if a full C Major triad is played or held continuously over a descending chromatic bassline starting on the note C, the resulting chords could be described as:
C C/B C/Bb C/A C/Ab etc.
You could also take account of the interval added by the bass note:
C Cmaj7/B C7/Bb C6/A
This is particularly useful if you want to "allow" musicians playing the full chords to include the bass note in the chords/lines they play.
Also, it is worth noting that some of the resulting chords can be described differently. For instance, a C/A or C6/A chord can also be named Am7. The choice of chord name is probably best decided by context. If you want to make it clear that the harmony is static above a changing bassline, it would be best to keep the chord name the same (i.e. use C/x in the examples above).