A combination is a grouping together of separate things. Your desire to live to a big city might come from a combination of your dreams of being on stage, and your obsession with underground trains.
Combination is the act of combining, which comes from the Latin for "joining together two by two," although it's not necessary that you combine things in pairs. Your famous party mix might be a combination of pretzels, nuts, and espresso beans. Your locker combination is the set of numbers you have to dial to open the lock. If you plan to go on a TV talent show, get ready to learn some dance combinations. Kick, spin, drop, roll!