A laptop computer or simply laptop (also notebook computer or notebook) is a small mobile personal computer, usually weighing from one to three kilograms, depending on size, materials and other factors.

While the terms "laptop" and "notebook" are often used interchangeably, "laptop" is the older term, introduced in 1983 with the Gavilan SC. "Notebook computer" is a later coinage, which was used to differentiate smaller devices such as those of the Compaq LTE series in 1989, which were, in contrast to previous laptops, the approximate size of an A4 paper sheet.As far as this distinction is concerned, the "thickness" ("vertical height") of the machine is disregarded; only its width and depth are considered Either term is often used improperly: due to heat and other issues, many laptops are inappropriate for use on one's lap, and most are not the size of an A4 sheet. Moreover, some older portable computers, such as the Macintosh Portable and certain Zenith TurbosPort models, were sometimes described as "laptops", although their size and weight were too great for this category.

Laptops usually run on a single battery or from an external AC/DC adapter which can charge the battery while also supplying the computer itself. As personal computers, laptops are capable of the same tasks as a desktop PC, although they are typically less powerful for the same price. 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 use different memory modules for their 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.