1. [techspeak] An error in which a running program attempts to access memory not allocated to it and core dumps with a segmentation violation error. This is often caused by improper usage of pointers in the source code, dereferencing a null pointer, or (in C) inadvertently using a non-pointer variable as a pointer. The classic example is:
scanf ("%d", i); /* should have used &i */
2. To lose a train of thought or a line of reasoning. Also uttered as an exclamation at the point of befuddlement.