On a string instrument, shifting, or a shift, is a movement of the fingers of the left hand from one position to another on the same string. Position is indicated through ordinal numbers (e.g., 3rd). Strings may be indicated through roman numerals, I-IV, and fingering may be indicated through numbers, 1-4. When done skillfully shifting avoids string noise.
In reference to classical guitar, "Fernando Sor recommends that one should 'be sparing of the operations called barring and shifting.'"[1]