Undefined terms in geometry are point, line, and plane. They are undefined terms because they are concepts and that either their dimensions or thickness can vary.
For example, a point is a location/position in space and it has no dimensions or thickness. In other words, a point has no specific size.
A line extends in both directions to infinity and also has no specific thickness.
All other terms in geometry are called defined terms because they rely on these three undefined terms, so a ray is defined because it uses undefined terms. A ray is defined as a portion of a line that starts at a point and goes to infinity in a particular direction.
