In agile methodologies, a retrospective is a moment in time in which the team stops to examine their work and improve their process and communication.
From the agile manifesto:
At regular intervals, the team reflects on how to become more effective, then tunes and adjusts its behavior accordingly.
Always follow the prime directive
Regardless of what we discover, we understand and truly believe that everyone did the best job they could, given what they knew at the time, their skills and abilities, the resources available, and the situation at hand. Norm Kerth
Five steps to a great retro: