A splinter was a fragment of a substance, usually wood, that became subcutaneously lodged in an individual's body.
In 2268, when attempting to express the immense difficulty of performing surgery to replace the Vulcan's brain in his skull, Spock said that he would trust Doctor McCoy to remove a splinter or lance a boil but not to attempt such a complicated procedure. (TOS: "Spock's Brain")
In 2372, while showing some Drayans around USS Voyager's sickbay, The Doctor assured First Prelate Alcia that they could perform "a wide variety of treatments here, from removing a splinter to re-sequencing the base pairs in a strand of DNA". (VOY: "Innocence")