Friction was a physical force created by two objects grating against each other. The term could also be used as a synonym for interpersonal conflict. (ENT: "A Night in Sickbay", TOS: "Elaan of Troyius", TNG: "Aquiel", "Second Chances", VOY: "Basics, Part I")
In 2266, James T. Kirk reported in his captain's log that the USS Enterprise's outer skin was heating quickly as friction built between it and the atmosphere of Psi 2000. (TOS: "The Naked Time")
Friction between two wooden sticks could lead to a buildup of heat energy, ultimately creating fire. (TNG: "Darmok"; ENT: "Desert Crossing"; VOY: "Basics, Part II")