What is a term for severe episodes that occur during REM sleep, similar to nightmares?

The term that describes severe episodes occurring during REM sleep, which are akin to nightmares, is indeed sleep terrors. Sleep terrors, also known as night terrors, typically manifest in a way that can involve intense fear, screaming, or thrashing about while the person is still asleep, often occurring without the individual being aware of their surroundings. These episodes are characterized by a sudden awakening with a high level of distress, and unlike nightmares, which are vivid dreams that can often be recalled, sleep terrors generally occur in non-REM sleep, complicating the experience.

In contrast, sleepwalking is a different phenomenon where an individual gets up and walks around while still asleep but is not connected with the intense fear associated with sleep terrors. Nightmares, while they can be very frightening and often occur during REM sleep, are usually more detailed, can be remembered upon waking, and are not as severe in terms of physical reactions as sleep terrors. Insomnia refers to difficulty in falling or staying asleep and not to episodes during sleep itself. Thus, sleep terrors accurately capture the described experience of severe episodes during sleep that resemble nightmares.