Thinking aloud is the concurrent verbalization of thoughts while performing a task (Ericsson and Simon, 1993). 

When this method is applied, participants are asked to spontaneously report everything that goes through their minds while doing a task, and they are instructed not to interpret or analyze their thinking. Verbal protocol is another term often used as a synonym for thinking aloud. Verbal protocols can be concurrent (thinking aloud) or retrospective, referring to short reports after the completion of a task.