The number assigned by the publisher to a separately issued part of a serial publication to distinguish it from other issues. Issue numbers may be assigned consecutively from the first issue onwards, but if the issues are divided into volumes, issue numbers recommence with each volume.
When an article published in a numbered issue of a periodical is cited, the issue number usually appears after the colon following the volume number. In the following example, the citation is to an article which appeared in the third number of Volume 9 of the journal Research Strategies, published in the summer of 1991:
O'Hanlon, Nancy. Begin at the End: A Model for Research Skills Instruction. Research Strategies 9:3 (Summer 1991), 116-123.