The Splinter is a small piece of wood that SpongeBob got when he was in the Krusty Krab at work and fell. When Patrick put trash from a dumpster onto it, it was swollen and turned purple with green puss. Squidward told Mr. Krabs about the splinter as he felt it would send SpongeBob back home. At the end of the episode, Mr. Krabs pulls it out, it explodes into confetti, and then into green pus at Krabs dismay.
The splinter only appears in the show with the episode with same name.