The concave term is a term that is used both in mathematics (especially geometry) and in physics to refer to a type of angle that is generated before a curve and that supposes the internal side of it, that is, where the internal cavity is generated. The opposite of the concave is the convex term, the outer side of the curve. Both terms are normally used as qualifying adjectives and can be used to designate different elements or objects in which this phenomenon occurs.
The etymology of the word concave is not entirely clear since while it is argued that it may come from the Latin term cavus or cavity, it is also believed that the Greek term kutos would give cavity. The idea of the concave concept is, ultimately and regardless of its origin, that of the presence of a cavity that becomes visible when a straight line is transformed into a curve separating the space into two semi-planes: one internal and one external to the curve.
When we talk about the internal plane of the curve, we are referring to that plane that is almost enclosed by the curve while the external one will be the one that is represented by everything outside. Thus, the internal plane will be transformed into a concave plane because since the curve is not a straight line, an imbalance will be generated between the two planes and one of them will have a cavity while the other will represent the curvature of the opposite side. In this sense, it is important to note that the word convex comes from Latin and means Carry on one's back, by which it is understood that the word represents the side that seems to be bent of the two that can generate a curve.
Both theoretical and abstract concepts are applicable to reality in a lot of objects in which we observe a curvature and the generation of these two planes, for example, the concave side of a lens.
