A ray is a part of a line that has one endpoint and extends infinitely in one direction:
This number line showing whole numbers is really not a line in geometry, but a ray. It begins at the endpoint, 0, and extends infinitely because there is no greatest whole number.
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A ray of sunshine begins at the sun (the endpoint) and travels through space in one direction. A ray from the sun can be used as a model for thinking about a ray in geometry.
See also lines, line segment.