turning point

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The point at which a very significant change occurs; a decisive moment.
• noun Mathematics A maximum or minimum point on a curve.

from The Century Dictionary.

• noun In graphics, a maximum or minimum point on the graph or curve.
• noun The point on which a thing turns; the point at which motion in one direction ceases and that in a contrary or different direction begins; the point at which a decisive change takes place, as from good to bad, from increase to decrease, or the opposite.
• noun In engineering, a temporary bench or bench-mark, the exact elevation of which is determined in leveling before the instrument is advanced, as a starting-point for determining its height after resetting.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A decisive point at which a significant change or historical event occurs, or at which a decision must be made.
• noun calculus A maximum or minimum on a graph.
• noun A crossroads.
• noun A T-junction.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the intersection of two streets
• noun an event marking a unique or important historical change of course or one on which important developments depend