Vertex
A corner point of a shape; 3D surfaces are built from many of them joined into triangles.
See it in Mesh warp. Turn on Show points, then drag one to bend the picture.
In plain words
A vertex is a corner. A triangle has three, and a square drawn from two triangles has four, because two are shared. Each vertex stores where it sits in space, and often more, such as a colour or which part of a picture belongs there. Moving vertices changes the shape without adding any new ones. The GPU handles all the vertices first, then fills in the pixels between them.