In the psychology subfield of oneirology, a lucid dream is a type of dream wherein the dreamer knows that they are dreaming while in their dream. The capacity to have and sustain lucid dreams is a trainable cognitive skill. During a lucid dream, the dreamer may gain some amount of volitional control over the dream characters, narrative, or environment, although this control of dream content is not the salient feature of lucid dreaming. An important distinction is that lucid dreaming is a distinct type of dream from other types of dreams such as prelucid dreams and vivid dreams, although prelucid dreams are a precursor to lucid dreams, and lucid dreams are often accompanied with enhanced dream vividness. Lucid dreams are also a distinct state from other lucid boundary sleep states such as lucid hypnagogia or lucid hypnopompia.