grandfather paradox

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

• noun A causality paradox speculated about in theories of time-travel in which traveling back in time would allow one to alter the conditions at the earlier time in such a way as to make current conditions impossible, as by causing the death of one's grandfather, making one's very existence impossible.

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

• noun uncountable The paradox of time travel that hypothetically occurs when one person goes back to the past and kills their biological grandfather before the latter fathered the traveller's father, thus, by extension, preventing the traveller to ever be conceived in the first place.
• noun countable An instance of the grandfather paradox.