A lattice of energy filaments woven around a target by pairs of Tholian ships, slowly enclosing it until the vessel inside can be held or destroyed.
The Tholian web is the signature tactic of the Tholian Assembly, first seen in the episode The Tholian Web. After Commander Loskene accuses the Enterprise of trespassing and an exchange of fire leaves the starship disabled, two Tholian vessels begin spinning strands of energy between them.
The ships work methodically, laying filament after filament until a complete cage surrounds the Enterprise. Spock calculates that once the structure is finished the ship will be unable to escape.
The crew survive because the unstable space of the region throws the Enterprise clear at the moment Kirk is beamed back aboard. The web is seen again in the Enterprise two-parter In a Mirror, Darkly, where Tholians of the mirror universe use one to trap and destroy the Terran starship Enterprise.