Resting Voltage and State of Charge (SoC)
The open-circuit voltage measured after the battery has been disconnected from load and charge and allowed to rest; in the lead-acid family it is the most practical way of estimating the state of charge. For a 12 V battery, around 12.7 V is typically taken as fully charged and around 12.2 V as roughly half charged.
For the measurement to be valid the battery must genuinely have rested. Right after charging stops there is surface charge on the plates and you read higher than the true value; right after a discharge you read lower. In lead-acid a rest of several hours is typically expected.
Temperature calls for a correction too. A voltage measured in the cold looks different for the same state of charge, so for serious measurements the manufacturer's temperature correction table is used. Resting voltage is therefore an estimate, not a precise gauge.
In LiFePO4 this method works much more poorly. The voltage curve of a lithium pack is almost flat in the middle region; between 13.2 V and 13.3 V the state of charge can correspond to a wide range. That is why a battery monitor that integrates current is preferred in lithium systems. The practical rule in an RV is this: voltage tells you the rough situation, but only a device that counts amp-hours tells you how much energy is left.
Context in which this term is used: RV automatic levelling guide