apex

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The highest point; the vertex: the apex of a triangle; the apex of a hill.
2. n. The point of culmination. See Synonyms at summit.
3. n. The usually pointed end of an object; the tip: the apex of a leaf.

Century Dictionary and Cyclopedia

1. n. The tip, point, or summit of anything. In botany: The end furthest from the point of attachment or base of an organ.
2. n. In geology, the top of an anticlinal fold of strata. [Pennsylvania coal-mines.] [This term as used in the U. S. Revised Statutes has been the occasion of much litigation. It is supposed to mean something nearly equivalent to outcrop (which see); but precisely in what it differs from outcrop has not been, neither does it seem capable of being, distinctly made out.]
3. n. In mining, the landing-point at the top of a slope.
4. n. In Roman antiquity, a symbolic ornament which the flamens and some other priests were required by law to wear. It consisted of a small cone of olive-wood surrounded with a lock of wool, and was secured on the head by fillets or adjusted to a cap.
5. n. In projective geometry, the point determined by 3 planes.
6. n. plural The abacusmarks. The apices of Gerbert's abacus are symbols for the digits from 1 to 9, but without the zero. See the extract.

Wiktionary

1. n. The highest point of something.
2. n. The moment of greatest success, expansion, etc.
3. n. The topmost vertex of a cone or pyramid (in their conventional orientation).
4. n. The pointed end of something.
5. n. The lowest part of the human heart.
6. n. The deepest part of a tooth's root.
7. n. The end of a leaf, petal or similar organ opposed to the end where it is attached to its support; the tip.
8. n. The point on the celestial sphere toward which the sun appears to move relative to nearby stars.

GNU Webster's 1913

1. n. The tip, top, point, or angular summit of anything
2. n. The end or edge of a vein nearest the surface.

WordNet 3.0

1. n. the highest point (of something)
2. n. the point on the celestial sphere toward which the sun and solar system appear to be moving relative to the fixed stars