A laptop computer, or simply laptop (also notebook computer or notebook), is a small mobile computer, which usually weighs 2.2-12 pounds (1-6 kilograms), depending on size, materials and other factors.

While the terms laptop and notebook are often used interchangeably, "laptop" is an older term, introduced in 1983 with the Gavilan SC. The term “Notebook computer” was coined later, and was used to differentiate smaller devices such as those of the NEC UltraLite and Compaq LTE series in 1989, which were, in contrast to previous laptops, the approximate size of an A4 paper sheet. Both terms are imprecise: due to heat and other issues, many laptops are inappropriate for use on one's lap, and most notebooks are not the size of typical A4 paper notebook.

Laptops usually run on a single battery or from an external AC/DC adapter capable of charging the battery while also supplying power to the computer itself. As personal computers, laptops are capable of the same tasks as a desktop PC, although they are typically less powerful than similarly priced.

They contain components that are similar to their desktop counterparts and perform the same functions, but are miniaturized and optimized for mobile use and efficient power consumption. Laptops usually have liquid crystal displays and most of them use different memory modules for their random access memory (RAM), for instance, SO-DIMM in lieu of the larger DIMMs. In addition to a built-in keyboard, they may utilize a touchpad (also known as a trackpad) or a pointing stick for input, though an external keyboard or mouse can usually be attached.

Many schools have taken in laptop based programs in which every student receives a laptop for school use only. The operating system is configured (or a third party program is installed) to limit the student's access to perform administrative tasks such as hardware or software installation, or operating system modifications. Students are generally permitted to use these systems to take notes, write papers, and perform other school-related activities.

A PDA is an extremely small and lightweight portable computer, with most of the features of a standard laptop computer and running all its applications. Its overall size is much smaller than an average notebook.

Hence it often has smaller than 10" (diagonal) screen and weigh less than 1 kg; as opposed to full-size laptop with 15" screen and weighing 2 kg or more. These savings in size and weight are usually achieved by omitting ports and/or removable media drives; subnotebooks are often paired with docking stations to compensate.

In 1996, the Toshiba Libretto 20 was first released with an AMD 486 processor, 6.1" screen and 270MB hard disk, weighing 840g and running Windows 95. In 1996, Sony released their best known N505 series VAIO subnotebook, which has been recognised as the master design and symbol of a machine in this size. With carbon fiber structured TX, SZ and X505 series, Sony is still a leader in the subnotebook market.

Recently, the subnotebook has lost the title to the Ultra Mobile PC as the smallest and most portable full-blown computer.