Wikipedia
The atomweight division in mixed martial arts generally refers to competitors weighing at or less than . It sits below the heavier strawweight division and is the lightest weight class widely recognized within MMA.
The atomweight division in mixed martial arts is not defined by the Unified Rules and is used almost exclusively for Women's MMA.
Invicta Fighting Championships, an all women's MMA organization, regularly holds contests at this weight and circulates a championship title. They held their inaugural atomweight title fight on October 6, 2012. The contest saw Jessica Penne defeating Naho Sugiyama by triangle choke in the second round. The title is currently held by Ayaka Hamasaki.
The ONE Championship's atomweight limits between 106 to 115 lb (47.7 to 52.2 kg)