Final answer:
In Euclidean geometry, at least three points on the same line are required to define a plane.
Step-by-step explanation:
According to Euclidean geometry, a plane contains at least three points that lie on the same line. This principle is foundational in Euclidean geometry, which assumes a flat space. When these three points are connected, they can form a triangle. In Euclidean space, a straight line is considered the shortest distance between two points, and the angles of a triangle always add up to 180 degrees. This fact is significant when understanding the nature of planes and lines in Euclidean geometry.