A chord is a line segment whose endpoints are on a circle. A diameter is a chord that passes through the center of a circle. Another one of the parts of a circle is a radius, which is a line segment with one endpoint at the center and one endpoint on the circle. Congruent circles have congruent radii (the plural of radius). Concentric circles have the same center. A central angle has a vertex on the center and endpoints on the circle.
The definition of a circle is the set of all points in the same plane that are a given distance from a given point.
So here we have a given point, our center, and the circle, which is the black line. And those are all the points that are a given distance away from the center. And that given distance is your radius. If you have a chord or a segment that passes through the center, that is called a diameter.
Now, two commonly confused terms regarding circles are congruent circles and concentric circles. Circles whose radii have the same measure are congruent. Circles who share the same center are concentric, which means the center of the smaller circle is the same center as the larger circle. The radiis are different, so they are not going to be congruent.
However, you could have concentric and congruent circles in which case I could say that here I've drawn two concentric and congruent circles because they are just going to overlap because they have the same center and the same radius.
So keep that in mind when you are answering questions, usually true and false and matching, about circles.