Tangent-1 refers to the inverse tangent function or arctangent. This function takes a value between negative infinity and positive infinity as the input and returns an angle in radians as the output. For example, if tangent(x) = -1, then tangent-1(-1) = -0.785 radians. This is approximately -45 degrees, which means that the angle whose tangent is -1 is -45 degrees or -0.785 radians.