The running of two or more programs in one computer at the same time. The number of programs that can be effectively multitasked depends on the sophistication of the operating system, the speed of the CPU and the speeds and capacities of memory and disk. See preemptive multitasking.

Input/Output Vs. Processing
Programs can be run simultaneously in the computer because of the difference between I/O and processing speed. While one program is waiting for input, instructions in another can be executed. During the milliseconds one program waits for data to be read from a disk, millions of instructions in another program can be executed. In interactive programs, thousands of instructions can be executed between each keystroke on the keyboard.

Channels and Multicore Provide Simultaneity
In mainframes and mainframe-like architectures, multiple I/O channels allow for simultaneous I/O operations to take place. Multiple streams of data, sometimes hundreds, are being read and written at the exact same time.

If a computer has more than one CPU, one program can be running in one CPU and another program in the second CPU, and so on (see multicore).

A Note on Ancient Terminology
In the days of mainframes only, multitasking was called "multiprogramming," and multitasking meant "multithreading." See multithreading.