Early 19th-century socialist thought of Saint-Simon, Fourier, Owen and Cabet, which proposed model communities and persuasion rather than class struggle. The label 'utopian' was applied later, often as criticism, by Marx and Engels.
In more detail
Britannica ties it to Robert Owen and Charles Fourier and counts John Humphrey Noyes among its advocates; settlements such as New Harmony, Brook Farm and the Icarian colonies are its best-known tests. Marx and Engels contrasted it with their 'scientific socialism', arguing that it appealed to all classes and rejected revolutionary action. Some later writers argue its founders were experimenters who tested socialism in small communities. The term is therefore both descriptive and polemical.