What is a lump on my gum with white puss in it?

An abscess: A lump on the gums is an abscess. Typically, there are 3 reasons for it. 1) endodontic - the pulp of the tooth is infected. Could be due to a cavity or trauma. 2) periodontic - the is an infection around the tooth. There is a space between the tooth and gums where bacteria can get trapped and abscess. 3) a fractured tooth. An abscess is treated with antibiotics and removal of the infection.