A transporter erector launcher (TEL) is a vehicle with an integrated prime mover that can carry, elevate to firing position and launch one or more missile

Missile

Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

s. Such vehicles exist for both surface-to-air missile

Surface-to-air missile

A surface-to-air missile or ground-to-air missile is a missile designed to be launched from the ground to destroy aircraft or other missiles...

s and surface-to-surface missile

Surface-to-surface missile

A surface-to-surface missile is a guided projectile launched from a hand-held, vehicle mounted, trailer mounted or fixed installation or from a ship. They are often powered by a rocket motor or sometimes fired by an explosive charge, since the launching platform is typically stationary or moving...

s. Early such missiles were launched from fixed sites and had to be loaded onto trucks for transport, making them more vulnerable to attack since once they were spotted by the enemy they couldn't easily be relocated, and if they were it often took hours or even days to prepare them for launch once they reached their new site.

A transporter erector launcher and radar (TELAR) is the same as a TEL but also incorporates part or all of the radar system necessary for firing the missile(s). Such vehicles have the capability of being autonomous, greatly enhancing their effectiveness. With this type of system each vehicle can fight regardless of the state or presence of support vehicles. The TEL or TELAR may have a rotating turntable that it can use to aim the missiles. The vehicle may have to turn to aim the missiles or they may fire straight up.

Conversely, a transporter launcher and radar (TLAR) is the same as a TELAR without the erector capability (presumably because the missile does not need to be erected for launch).

Usually a number of TELs and TELARs are linked to one Command post vehicle (CP or CPV). They may utilise target information from Target acquisition, designation and guidance radar (TADAGR) or, simply, TAR.

The Patriot missile system

MIM-104 Patriot

The MIM-104 Patriot is a surface-to-air missile system, the primary of its kind used by the United States Army and several allied nations. It is manufactured by the Raytheon Company of the United States. The Patriot System replaced the Nike Hercules system as the U.S. Army's primary High to Medium...

uses the abbreviation MEL (Mobile Erector Launchers) as a towed launch vehicle.