A touchpad (also trackpad) is a pointing device consisting of specialized surface that can translate the motion and position of a user's fingers to a relative position on screen. They are a common feature of laptop computers and also used as a substitute for a computer mouse where desk space is scarce. Touchpads vary in size but are rarely made larger than 40 square centimeters (about 6 square inches). They can also be found on personal digital assistants (PDAs) and some portable media players, such as the iPod using the click wheel. Also, it is a new type of Hanvon multitouch device. A touchpad is perhaps the most common kind of tactile sensor.

Touchpads operate in one of several ways, including capacitive sensing and conductance sensing. The most common technology used today entails sensing the capacitance of a finger, or the capacitance between sensors. Because of the property being sensed, capacitance-based touchpads will not sense the tip of a pencil or other similar implement. Gloved fingers will generally also be problematic (such as in a cleanroom environment).

While touchpads, like touchscreens, by their design are able to sense absolute positions, precision is limited by their size. For common use as a pointer device, the dragging motion of a finger is translated into a finer, relative motion of the cursor on the screen, analogous to the handling of a mouse that is lifted and put back on a surface. The buttons are below, above or sideways — the latter is often used for compact devices, such as Netbooks — for the pad serves as standard mouse buttons. Depending on the model of touchpad and drivers behind it, a user may also click by tapping their finger on the touchpad, and dragging it with a tap followed by a continuous pointing motion (a "click-and-a-half").Touchpad drivers can also allow the use of multiple fingers to facilitate the other mouse buttons (commonly two-finger tapping for the center button).

Some touchpads also have "hotspots": locations on the touchpad that indicate user intentions other than pointing. For example, on certain touchpads, moving the finger along an edge of the touch pad will act as a scroll wheel, controlling the scrollbar and scrolling the window that has the focus vertically or horizontally depending on which edge is stroked. Apple uses two-finger dragging gesture for scrolling on their trackpads. However, these are driver dependent functions and can be disabled. Also, certain touchpad drivers allow for tap zones, regions whereby a tap will execute a function. For example, pausing the media player or launching an application.

arly Apollo desktop computers were equipped with a touchpad on the right side of the keyboard.

Touchpads are primarily used in portable laptop computers, because a standard mouse requires a flat surface near the keyboard not always available outside of a standard computing environment. Because the touchpad's position is fixed relative to the keyboard, and very short finger movements are required to move the cursor across the display screen, many users find touchpads preferable,[original research?] and desktop keyboards with built-in touchpads are available from specialist manufacturers. However, these features can also cause frustration when a user's thumb accidentally swipes over the touchpad while typing.

One-dimensional touchpads are also the primary control interface for menu navigation on all of the currently produced iPod portable music players (except the iPod shuffle and iPod Touch), where they are referred to as "click wheels" since they only sense motion along one axis which is wrapped around like a wheel. Creative Labs also uses a touchpad for their Zen line of MP3 players, beginning with the Zen Touch. The second-generation Microsoft Zune product line (the Zune 80/120 and Zune 4/8) uses touch for the Zune Pad.

Apple's PowerBook 500 series was the first laptop to carry such a device, which Apple refers to as a "trackpad". When introduced in May 1994, it replaced the trackball of previous PowerBook models. Apple's more recent laptops feature trackpads that can sense up to five fingers simultaneously, providing more options for input, such as the ability to bring up the context menu by tapping two fingers. In late 2008 Apple's revisions of the MacBook and MacBook Pro feature a different design to the touchpad in which the button is incorporated into the tracking surface.

Psion PLC's Psion MC 200/400/600/WORD Series, introduced in 1989, came with a new mouse-replacing touchpad;[5] however, the Psion's device more closely resembles a graphics tablet than a touchpad, as one would position the cursor by clicking on a specific point on the pad, instead of moving it in the direction of a stroke.

There are two principal means by which touchpads work. In the matrix approach, a series of conductors are arranged in an array of parallel lines in two layers, separated by an insulator and crossing each other at right angles to form a grid. A high frequency signal is applied sequentially between pairs in this two-dimensional grid array. The current that passes between the nodes is proportional to the capacitance. When a virtual ground, such as a finger, is placed over one of the intersections between the conductive layer some of the electrical field is shunted to this ground point, resulting in a change in the apparent capacitance at that location. This method received U.S. Patent 5,305,017 awarded to George Gerpheide in April 1994.

The capacitive shunt method, described in an application note by Analog Devices, senses the change in capacitance between a transmitter and receiver that are on opposite sides of the sensor. The transmitter creates an electric field which oscillates at 200-300 kHz. If a ground point, such as the finger, is placed between the transmitter and receiver, some of the field lines are shunted away, decreasing the apparent capacitance.

Scrolling TrackPad is the name for one of Apple Inc.'s patent-pending trackpads, used in their MacBook and MacBook Pro laptop computers. They were previously used in the PowerBook and iBook lines, prior to Apple's switch to Intel processors. It lets users scroll in an arbitrary direction by touching the pad with two fingers instead of one, and then moving their fingers across the pad in the direction they wish to scroll. A feature introduced in Mac OS X 10.5 (Leopard) allowed users to do various other combinations of gestures such as swiping four fingers up or down to activate exposé. There are also many applications available which are designed to extend the multi-touch options. For comparison, many laptop touchpads instead set aside an area along the right edge and bottom edge of the pad, and moving a single finger in these areas performs a vertical or horizontal scroll operation, respectively. Current MacBooks, MacBook Pros and MacBook Airs have a Multi-Touch pad. Apple's newest multitouch trackpads also support pinching and rotating gestures, similar to those used on iPhone devices.