Imperfect self-defense arises when an individual honestly, but unreasonably, believes that they need to use force to defend themselves from an imminent threat. Unlike perfect self-defense, which can fully exculpate the defendant if it was reasonable and proportional to the actual threat, imperfect self-defense does not negate guilt entirely but can reduce the severity of the charge (e.g., from murder to voluntary manslaughter).
For example: Suppose a person is approached aggressively but unarmed, yet they genuinely believe the aggressor has a weapon and they fatally shoot them. This might be considered imperfect self-defense, as the use of deadly force was disproportionate given the actual situation.