When two things are interchangeable, you can use one in place of the other and nothing important breaks. A spare part is interchangeable if it fits the same way as the old part. Two workers can be interchangeable if either can do the job. With words, English speakers often say two words are interchangeable when they are close in meaning. Though actually, calling people interchangeable can sound a little cold — as if they do not matter as people, only as roles. And with words, “interchangeable” is often used a bit loosely: the two words can swap in many places, but not in every place. The word is fairly neutral; you meet it in shops, factories, and careful writing. The usual pattern is interchangeable with, not interchangeable to.