These terms all relate to having people sleep over in a different house, but they don’t mean the same thing. A sleepover can include as little as two people and is rarely as planned as slumber or pajama parties. Pajama parties and slumber parties are very similar because both of them involve multiple people, but only one requires you to strictly wear pajamas. A lock-in also involves multiple people, but unlike the others, it is typically not held in a person’s home.