The Vertex is a fifth chart angle alongside the Ascendant, Descendant, Midheaven, and Imum Coeli — a mathematical point derived from the exact birth time and location, not a body in the sky. It marks where the ecliptic crosses the prime vertical, the great circle running through the exact east and west points of the horizon and straight up through the zenith overhead.
Astrologically it's associated with encounters and turning points that feel less chosen than fated — meetings, relationships, or events that arrive from outside and reroute something. It's a subtler point than the four main angles, usually most noticeable when a transit or another person's planet lands directly on it.
Like the Ascendant, the Vertex depends on knowing an accurate birth time — without one, it can't be calculated.