A danger. The peril must be real and substantial, but it need not be imminent.
The distinction between action taken for the common safety in time of peril and a measure which, however reasonable, is purely precautionary, is a very fine one. A ship drifting without engine power in mid-ocean would certainly be in peril under the York-Antwerp Rules (which govern the assessment of most general average cases), even though the weather might be calm at the time and there was no immediate threat. Sooner or later, ship and cargo would come to grief one way or another, so the cost of a tow to safety would therefore qualify in general average. Where the master of a perfectly sound ship prudently decides to run for shelter from an approaching storm, there is no general average act, since the measure is purely a precaution of a prudent seaman.