Wiktionary
n. (context physics English) A two-dimensional manifold which describes the embedding of a string in spacetime, a direct generalization of the worldline of a particle in special relativity and general relativity.
Wikipedia
In string theory, a worldsheet is a two-dimensional manifold which describes the embedding of a string in spacetime. The term was coined by Leonard Susskind around 1967 as a direct generalization of the world line concept for a point particle in special and general relativity.
The type of string, the geometry of the spacetime in which it propagates, and the presence of long-range background fields (such as gauge fields) are encoded in a conformal field theory defined on the worldsheet. For example, the bosonic string in 26-dimensional Minkowski space has a worldsheet conformal field theory consisting of 26 free scalar fields. Meanwhile, a superstring worldsheet theory in 10 dimensions consists of 10 free scalar fields and their fermionic superpartners.
Usage examples of "worldsheet".
That is, their two-dimensional worldsheet can fully surround a two-dimensional sphere, as in Figure 11.