Equivalent Oxide Thickness (EOT), represented by teq or tOX, is the gate oxide thickness of the SiO2 layer of a transistor that would be required to achieve similar capacitance density as the high-κ material used.
A gate dielectric with a dielectric constant that is substantially higher than that of SiO2 will initially have a much smaller equivalent electrical thickness. This key feature allowed for the industry to continue on with Moore's Law. As the semiconductor industry began to experiment with transitioning from a SiO2 gate oxide to a high-κ material, EOT can be used to quickly compare those materials using existing SiO2-based models.
For example, consider Hafnium Dioxide (HfO2) which has an (subject to variations in temperature). A layer of just 6.15 nm in thickness Hafnium Dioxide (HfO2) would result in an equivalent SiO2 oxide thickness of around . This is indeed the material used by Intel following their transition to high-κ at the 45 nm process node.