It is generally not advisable to reference abstracts. This is especially the case if the full article is available, and referencing the abstract should not be used as a substitute for reading the source. Always consult with your professor before citing from an abstract — some professors do not allow you to do so. In most citation styles, use the format of the source from which you are taking the abstract and incorporate the term "Abstract" into the reference.
An abstract of an online journal article using the APA style would read as follows:
Denny, H., Nordlof, J., & Salem, L. (2018). "Tell me exactly what it was that I was doing that was so bad": Understanding the needs and expectations of working-class students in writing centers. Writing Center Journal [Abstract], 37(1), 67. https://www.jstor.org/stable/26537363
An abstract of a journal article using MLA style would read as follows:
Ong, Yi-Ping. Abstract of “Anna Karenina Reads on the Train: Readerly Subjectivity and the Poetics of the Novel.” PMLA, vol. 133, no. 5, Oct. 2018, p. 1302.
An abstract of an online journal article using Chicago Notes-Bibliography style would read as follows:
Matuate, Daniel R. "Noisey Neighbors Can Hamper the Evolution of Reproductive Isolation by Reinforcing Selection." Abstract. American Naturalist 185, no. 2 (February 2015): 253–69. https://doi.org/10.1086/679504.