Wikipedia
Autonegotiation is an Ethernet procedure by which two connected devices choose common transmission parameters, such as speed, duplex mode, and flow control. In this process, the connected devices first share their capabilities regarding these parameters and then choose the highest performance transmission mode they both support. In the OSI model, autonegotiation resides in the physical layer. For Ethernet over twisted pair it is defined in clause 28 of IEEE 802.3.
http://standards.ieee.org/getieee802/download/802.3-2012_section2.pdf
IEEE 802.3, clause 28, page 221, "Physical Layer link signaling for Auto-Negotiation on twisted pair"
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Autonegotiation was originally defined as an optional component in the fast Ethernet standard. It is backwards compatible with 10BASE-T. The protocol was significantly extended in the gigabit Ethernet standard, and is mandatory for 1000BASE-T gigabit Ethernet over twisted pair.