Which term describes a round that detonates on impact?

The term that describes a round detonating on impact is "graze." In artillery terminology, a graze round is designed to explode upon striking a target, ensuring maximum effectiveness against ground targets. This type of detonation is crucial in for target engagement, as it typically leads to immediate damage upon contact, thereby providing a more effective means of engagement for both direct fire and indirect artillery support.

The other terms listed do not specifically relate to rounds detonating on impact: air refers to munitions designed to explode in the air for broader area effects; mixed air implies a combination of detonation types; and doubtful refers to scenarios where the effectiveness of the round cannot be guaranteed.