Triangulation

One of the two major methods of Wi-Fi localization (Wi-Fi fingerprinting being the other). In its purest form, triangulation uses angles to calculate location based on known distant reference points, which requires clear line of sight (LoS). When indoors, LoS is a rarity, and site-specific signal propagation models are used.

Rather than being distinguished from triangulation, lateration methods are considered a subset of triangulation methods.

angulation
Uses angles.

Angle of Arrival (AOA)
Requires clear line of sight and reliable device orientation. Requires special hardware. Also Direction of Arrival (DOA) or Direction Finding (DF).

lateration
This is the traditional method used by cell-phone (mobile network) location systems.

Time of Arrival (TOA)
A lateration method. Requires a site-specific signal propogation model unless used with clear line of sight in a vaccuum. Also requires precisely synchronized receivers and transmitters, making it no good for uncontrolled environments.

Time Difference of Arrival (TDOA)
Another lateration method. Requires that the measuring units all share a precise time reference. Even were that not required, requires that the positioning software be on the APs and again it is no good for general purpose use.

Received Signal Phase Method
needs LOS signal path

Return Time of Flight (RTOF)
Also known as Round-trip Time of Flight. Another lateration method. Requires only moderate time synchronization. Assuming you can make your device send a packet that you can get back and measure the time, the difficulty here is in knowing the processing time. In unknown environments, this is difficult. Even in a known environment, AP models can vary and loads can determine response time.

trilateration
Lateration in two dimensions, requiring three stationary antennas. Uses overlapping circles centering on each BS to determine MS location; looks like a venn diagram.

multilateration
In this context, lateration in three dimensions. Like trilateration, but using spheres instead of circles, and requiring at least four stationary antennas.