Elvish for "petty-dwarves". In The Silmarillion, they are dispersed and secretive dwarves who entered Beleriand before the elves made contact with the dwarves of Belegost and Nogrod. They were persecuted and hunted down because the elves thought they were creatures of Morgoth, and as a result they became accustomed to hiding in hidden underground homes, and also to hating the elves (especially the Noldor).
The only petty-dwarves to play a major role were Mim and his sons Khim and Ibun. They got into a fight with Turin and his companions, and Turin fatally shot Khim with an arrow and captured Mim. Mim took Turin to his home in return for mercy, but later betrayed Turin to orcs when they captured him. He was later slain by Beleg. His home, on the mountain Amon Rudh, was known as Bar-en-Danwedh, the House of Ransom.
Mim and his sons are the only individual dwarves to play any significant role in the Silmarillion.