Drift
Drift turns up in 2 fields: statistics, metrology. Here is what it means to the people in each one.
Drift in statistics
Slow, steady creep of a process or gauge reading in one direction as something wears, depletes or ages.
Drift is gradual change over time: an etch rate falling as chamber walls coat, a polishing pad glazing, a sensor aging, a filament thinning, a reference standard degrading. On a control chart it shows up as a trend or a long run before any single point breaks a limit. Run-to-run controllers and EWMA charts are designed to track it, and some scheduled maintenance exists purely to reset it.
"Deposition rate drifts down about half a percent a day until the next wet clean, so the controller bumps time to compensate."
Drift in metrology
Gradual change in an instrument's readings over time that comes from the instrument itself, not from what it measures.
A change in what an instrument reads over time, steady or in jumps, that comes from the instrument itself, such as a Zener reference wandering a ppm or two per year or a pressure transducer's zero creeping as its sensor ages. Drift is estimated from a run of calibration results and enters the budget as a separate component covering the time since the last calibration. Predictable drift can be corrected; erratic drift shortens the calibration interval.
"Plot the last five cals on the 10 volt, the drift is a steady plus 0.8 ppm a year, so we can predict it."