A mini-bomb was a type of miniature explosive used by Starfleet officers in the 23rd century. It could be concealed in a hollowed uniform button.
In 2266, James T. Kirk used his mini-bomb to destroy a locked door, freeing the other members of the USS Enterprise landing party on Mila Xa. Later, Leonard McCoy used his mini-bomb to destroy a wall, uncovering an elevator shaft. (TOS comic: "The Haunted Asteroid")