A chord of a circle is a line segment with both endpoints on the boundary.
In the figure, and are chords of the circle.
A diameter is a chord which passes through the center of the circle and is the longest possible length for that circle.
Some basic theorems involving chords:
In the same circle or in congruent circles, two minor arcs are congruent if and only if their corresponding chords are congruent.
In the same circle or in congruent circles, two chords are congruent if and only if they are equidistant from the center.