Suppression is the act of keeping something from happening — like the suppression of tears while sitting on a public bus.
In psychology, suppression is the act of stopping yourself from thinking or feeling something. It is generally presumed to be ineffective because even if you suppress or hold back an emotion, like anger, that feeling returns with a vengeance. Suppression can also mean the act of holding something else back, like the suppression of a book from publication because you don't like its message.