Definition of "unidirectional"

[...] a reversible booster was fitted to the trailing bogie; all previous applications on the L.N.E.R. for use in main line traffic had been of unidirectional boosters, [...].

1941 November, O. S. Nock, “The Locomotives of Sir Nigel Gresley: Part VI—Experiments and Rebuilds 1927-33”, in Railway Magazine, page 483

Equally important is the class mailing list, which facilitates two-way communication at a distance. (By contrast, the web page is basically a unidirectional teacher-to-student communication.)

2005, Richard Samson, edited by Martha Tennent, Training for the New Millennium: Pedagogies for translation and interpreting, Benjamins Translation Library, page 119