Highlight the context clues that readers can use to determine what event horizon means. Around this extreme zone is a boundary called the event horizon. This boundary acts like a point of no return. Once past it, there's no going back. Only black holes have such extreme gravity zones, and only black holes have event horizons. This is what makes a black hole different from everything else. —A Black Hole Is NOT a Hole, Carolyn Cinami DeCristofano What does event horizon mean? boundary of no gravity boundary of extreme gravity horizon of the sun setting the line that separates earth from sky