breaking point

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

• noun The point at which physical, mental, or emotional strength gives way under stress.
• noun The point at which a condition or situation becomes critical.

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

• noun mechanics The point at which the increasing strain in a material causes it to break
• noun figuratively A moment of crisis, particulary one precipitated by a "last straw"

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

• noun (psychology) stress at which a person breaks down or a situation becomes crucial
• noun the degree of tension or stress at which something breaks