In geometry, an apex (Latin for 'summit, peak, tip, top, extreme end') is the vertex which is in some sense the "highest" of the figure to which it belongs. The term is typically used to refer to the vertex opposite from some "base."
In a pyramid or cone, the apex is the vertex at the "top" (opposite the base). In a pyramid, the vertex is the point that is part of all the lateral faces, or where all the lateral edges meet. Wikipedia, Apex
"For many years the necessity for a zero in a quantitative universe has been considered as necessary as a fulcrum of non-motion has been necessary for the expression of motion. The apex of the universal cone is as far as one can go in that direction, but whatever of infinite extension there is in this universe is in the direction of its base." [Atomic Suicide, page 250-251]
"To more fully comprehend this omnipresent radar universe of reflecting light mirrors and lenses, take one of the eight reflectors out of a cube, or make one composed of three mirrors placed the way they are shown in the corner of Fig. 62. If you analyze this shape you will see that it is half a cube cut through its diagonals. If you now slowly insert an object on the end of a long needle, toward the apex of that pyramid you will see it reflected three times, and those reflections will grow ever closer as your object nears that apex. That illustrates the multiplication, or compressive power of Nature. It also symbolizes centripetal force and higher potential. It also represents three of the octave elements on the red or blue side of the whole octave, of which the three mates are in the reverse corner reflector." [Atomic Suicide, page 258]