Theta (uppercase Θ or ϴ, lowercase θ (which resembles digit 0 with horizontal line) or ϑ; , thē̂ta, ; Modern: , thī́ta, ; , ) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth . In the system of Greek numerals it has a value of 9.

Any-angle path planning algorithms search for paths on a cell decomposition of a continuous configuration space (such as a two-dimensional terrain).