The time at which an ingested substance is at its highest possible concentration in the bloodstream. Used colloquially in the context of recreational drugs, most commonly psychedelics. When peaking, a psychedelic trip is usually at its most intense/powerful. The nature and duration of a peak varies by drug, user, and mindset at the time of tripping. Contrary to what some may say, it is not a term specific to any one particular drug.
Tripper A: "Dude... I..."
Tripper B: "What, man?"
Tripper A: "I'm... it's... lattitude."
Tripper B: "He's peaking hard."