pan-pan

"Pan-pan" actually derives from the French "panne" which roughly translates to "failure" ("en panne" meaning "out of order"). Radio conventions make use of the word "Panne" (pronounced as in a cooking pan), repeated three times, to issue warning of an emergency situation related to some mechanical breakdown. "Panne-Panne-Panne" can then be upgraded to "Mayday" or even "Aaargh" as the situation unfolds.

Useful for clearing the landing runway, but it doesn't help a bit getting the dying propeller back to life.

For U.S. pilots, "pan-pan" is used as a specific indicator of distress that is (so far) non-life-threatening, for which "mayday" is reserved. For example, a VFR pilot who flies into cloud is trained to immediately perform a 180-degree instrument turn to regain clear air. If that fails (i.e. if the clouds are moving fast enough that the pilot becomes lost and/or disoriented and cannot figure out which way to fly to regain visibility) then one option is to call "pan-pan" on any ATC frequency. Any controller listening should immediately prioritize your request, so, for example, if you needed a radar fix in order to escape cloud cover, they could give you a bearing. In general, it means "I'm in some trouble and need help" but typically means that if you get said help you will be able to regain control/return to safe flying.

In the U.S., the phrase is "pan-pan" (two syllables, not three) although the phrase itself can be repeated. Typically, it (like mayday) is called three times. This phrase is intended to sound distinctly different from 'normal' radio traffic, as is "mayday", so that when repeated, it will be clear even over marginal radio links that the speaker is in distress.