What is a one movement musical work called?

What is a one movement musical work called?
This answer pertains to western classical music.

There is no singular term for this. Sorry.
Many musical forms are in one movement. To further complicate things, a particular form may change definitions over time. Thus, was was a multi-movement work in one period may be in a single movement 100 years later. Still more, as composers' individuality became more and more fore-fronted, they could individually violate standard definitions if they wanted to.
All that said, symphonic poems and tone poems often come in a single movement and they are often "through composed." Many through composed music is in a single movement (fantasia, fugue, for example), although they don't have to be. Many sectional works (in binary form, ternary form, strophic, etc) can be single-movement, although they are often joined with other sectional forms to form a multi-movement work.