Hello Chris, These two words are very close in meaning. If something is "defective", it means that it has a "defect", i.e., a fault, an imperfection, or a lack of something necessary for completion or perfection. If something is "deficient", it means that it is lacking in something. Generally speaking, we use "defective" if we are referring to something made up of discrete or distinct components, while we use "deficient" if we are referring to a property or quality that is scalar. So, my television may be "defective" if it is missing a specific part, or if a specific part does not work, but not usually "deficient" because it makes no sense to say that my television is not enough. By the same token, my vitamin A intake may be "deficient", in the sense that I am not eating enough carrots, but is extremely unlikely ever to be "defective" because it does not really mean anything to say that my vitamin A intake is missing a specific component. I hope this makes some sort of sense, Alvin