In systems that use virtual memory, the resulting condition of a hard drive being used excessively for virtual memory because the physical memory (i.e., RAM) is full. (The process of moving data into and out of virtual memory also is called swapping pages.) Disk thrashing considerably slows down the performance of a system because data has to be transferred back and forth from the hard drive to the physical memory.
A sure sign that your computer is thrashing is when an application stops responding but the disk drive light keeps blinking on and off. Thrashing is generally caused by too many processes competing for scarce memory resources. To temporarily stop thrashing, you need to terminate one or more applications. To stop it permanently, you need to install more main memory.