1. Interest rates - reference rates.
A 'fallback' is a specified alternative reference interest rate, for use in the event that the originally envisaged reference rate is unavailable.
"Whilst fallbacks are contained in existing documentation should a reference rate become (temporarily) unavailable, these were not drafted as a long-term solution [to the permanent retirement of LIBOR]."
ACT Briefing Note, Transition to risk free rate benchmarks.
Similar arrangements in other contexts.