What Is Thrashing In Operating System?

This is when you're trying to manipulate large amounts of data and the computer cannot fit them all into memory - it has to keep dropping some of the data out to the paging file (a virtual memory area which is actually just a file on disk) and then loading other bits back into memory.....if this movement is required a great deal, because you've got a lot of data, then the computer ends up spending more time moving data than actually processing it = thrashing

Thrashing occurs when a processor spends so much of its time transferring pages between virtual memory (swap file on disk) and physical memory (RAM) that it has little time to do anything else. It is often a sign that a user is trying to do more things at the same time than the physical memory of his/her computer can realistically sustain.