OCV
Open-circuit voltage
The battery's voltage at rest, neither charging nor discharging. Its relationship to charge is the 'fingerprint' that betrays the chemistry and the cells' state.
OCV is the voltage a cell holds when you leave it still, with no current going in or out. Each chemistry draws its own OCV curve as charge changes: that curve is almost a fingerprint.
On nickel chemistries (NMC/NCA) the curve is steep, so resting voltage betrays charge fairly well. On LFP it's flat in the middle, and there the trick stops working — which is why those cars need to charge to 100% now and then to recalibrate.