A scholarly periodical which requires that each article submitted for publication be judged by an independent panel of experts (scholarly or scientific peers). Articles not approved by a majority of these peers are not accepted for publication by the journal.
Peer-reviewed journals can be identified by their editorial statements or instructions to authors (usually in the first few pages of the journal or at the end). Many databases allow you to search for only "peer-reviewed" or "scholarly" articles.
Note: often your university instructors will expect you to use scholarly, peer-reviewed journal articles. However, if you are not sure about the acceptability of an article, website, or other source get in touch with your faculty member.