A circle is the set (locus) of all points in a plane that are at a fixed distance (radius) from a fixed point called the center.
Standard Equation (Center at (a, b), radius r):
\[ (x - a)^2 + (y - b)^2 = r^2 \]
Special Case (Center at origin (0, 0)):
\[ x^2 + y^2 = r^2 \]