An event-related potential (ERP) is any stereotyped electrophysiological response to an internal or external stimulus. In simple terms it is any measured brain response that is the direct result of a thought process or perception.
ERPs can be reliably measured using electroencephalography (EEG). This method utilizes surface electrodes to measure the electrical activity of the brain (specifically the cortex) through the skull and scalp. As the EEG reflects many thousands of simultaneously ongoing neuronal processes, the brain’s response to a specific stimulus or event of interest is rarely visible in the ongoing EEG. In actual recording situations, even the most robust ERPs emerge only after many dozens of individual presentations of the stimulus of interest are averaged together. This technique cancels out noise and spontaneous EEG and enhances the voltage response to the stimulus making it stand out clearly from the averaged out background.