"A preprint is defined as a draft, or version, of a paper that is posted on a preprint server and which has not yet been peer reviewed for formal publication. By definition, a preprint intends to disseminate research before acceptance and publication in a journal." from European Chemical Society Publishing.
"A preprint is a paper that is made available publicly via a community preprint server prior to (or simultaneous with) submission to a journal." from Wiley.
"A preprint is a piece of research that has not yet been peer reviewed and published in a journal. In most cases, they can be considered final drafts or working papers." from Preprints.org