* Line: This is the most likely answer. A line is a straight, continuous path that extends infinitely in both directions. It's one of the fundamental objects in geometry.
* Line segment: A line segment is a part of a line that has a defined beginning and end.
* Number line: This is a visual representation of numbers as points on a straight line. It's often used in arithmetic and algebra.
* Line of symmetry: This is a line that divides a figure into two identical halves.
Could you provide more context? What were you reading or hearing when you encountered this term? Knowing the context can help me understand what you're asking and provide a more accurate answer.