insideinside1preposition, adverb, adjectivein, on or to the inside of something: What's inside the box? It's raining – let's go inside (= into the building).the inside pocket of a jacketLook at outside.insideinsideinsides2nounthe part near the middle of something: The door was locked from the inside. There's a label somewhere on the inside.Look at outside.inside outwith the wrong side on the outside: You've got your jumper on inside out.