You may have heard the term 802.3 used in place of or in conjunction with the term Ethernet. "Ethernet" originally referred to a networking implementation standardized by Digital, Intel and Xerox. (For this reason, it is also known as the DIX standard.)
In February 1980, the Institute of Electrical and Electronics Engineers, or IEEE (pronounced "I triple E"), created a committee to standardize network technologies. The IEEE titled this the 802 working group, named after the year and month of its formation. Subcommittees of the 802 working group separately addressed different aspects of networking. The IEEE distinguished each subcommittee by numbering it 802.X, with X representing a unique number for each subcommittee. The 802.3 group standardized the operation of a CSMA/CD network that was functionally equivalent to the DIX Ethernet.
Ethernet and 802.3 differ slightly in their terminology and the data format for their frames, but are in most respects identical. Today, the term Ethernet refers generically to both the DIX Ethernet implementation and the IEEE 802.3 standard.