Cathode
Definition
The electrode where reduction happens. Remember Red Cat: Reduction = Cathode.
Technical definition
The cathode is the electrode at which reduction occurs in an electrochemical cell. In galvanic cells it is the positive electrode, while in electrolytic cells it is the negative electrode: the sign flips, but reduction always happens at the cathode.
Example
In a Daniell cell, the copper electrode is the cathode: Cu^2+ ions gain electrons and plate out as copper metal.
Common confusion
Like the anode, the cathode's plus/minus sign depends on cell type. In a rechargeable battery the same physical electrode is the cathode while discharging but the anode while charging.