To redial is to call a telephone number again — the same number you just tried. Many phones have a redial button that does this in one press, so you do not type the digits a second time. You redial when the line is busy, when nobody picks up, or when the call drops. It is an everyday verb, a little old-fashioned now, because so much calling is tapping a name, not turning a dial; but the word is still on phones and in instructions. You will also hear it as a noun: the redial, meaning the button or the act. It is neither formal nor slang. This is not a biology word and not “about a redia.”