A positive and negative battery terminal supplied as a pair, so both posts get a fresh, properly clamping connection at the same time. Corroded or spread terminals are a classic cause of hard starting, dim lights and intermittent electrical faults, and they are often blamed on the battery itself. Replacing both sides removes the doubt. Also a sensible upgrade when you fit a larger battery, relocate it to the boot, or add heavier cable for a high-output alternator or an electric fan and fuel pump load. Disconnect the negative first when removing the old terminals and reconnect it last, and make sure the cable is clean and bright where it clamps.