An abbreviation for Synchronization Ratio. It is a numerical representation of the level of synchronization between an Evangelion Unit and it's pilot. A higher Sync Ratio allows for better and more accurate control during combat, but since physical damage to an Evangelion Unit is felt by the pilot as physical trauma/pain, a higher Sync Ratio also leaves the pilot more sensitive to any damage the unit may sustain.
For much of the series, the highly-trained Asuka Langley Sohryu maintains the highest Sync Ratio, followed by Shinji Ikari, and Rei Ayanami. Eventually however, Shinji surpasses even Asuka.
The highest Sync Ratio on record is just over 400%, and was achieved by Shinji Ikari and a berserk Evangelion Unit-01 during the battle with Zeruel, the Fourteenth Angel. However this impossibly high Sync Ratio had breached Shinji's ego-barrier to the point that he was inadvertently absorbed into the Core of Evangelion Unit-01, where he stayed for over a month.