A momentary power outage is a:

A momentary power outage is a fault.

Power Excess

Spike --> Too much voltage for a short period of time.

Surge --> Too much voltage for a long period of time.

Power Loss

Fault --> A momentary power outage.

Blackout --> A long power interruption.

Power Degradation

Sag or Dip --> A momentary low voltage.

Brownout --> A prolonged power supply that is below normal voltage.

Reference(s) used for this question:

HARRIS, Shon, All-In-One CISSP Certification Exam Guide, 3rd. Edition McGraw-Hill/Osborne, 2005, page 368.

and

https://en.wikipedia.org/wiki/Power_quality