NKVD troika or Special troika (Russian: особая тройка), in Soviet Union history, were commissions of three persons who convicted people without trial. These commissions were employed as an instrument of extrajudicial punishment introduced to circumvent the legal system with a means for quick execution or imprisonment. It began as an institution of the Cheka, then later became prominent again in the NKVD, when it was used during the Great Purge. Troikas are credited with the death sentences or exile of over 600,000 people.
'Troika' literally means "a group of three" in Russian, like English "triad."
Read more about NKVD Troika: Background, Repression of Kulaks, Repression of Poles, Abolition