The google definition of a macro lens is "a lens that is suitable for taking photographs unusually close to its subject." However, what truly defines the lens is its magnification ratio.
The magnification ratio compares the size of the image captured by the camera sensor to the actual size of the object being photographed. This means an optic with a 10:1 ratio can project an image up to 10x the size of the object. This can be compared to a macro lens with a 1:2 ratio which then projects only half the size of the object.
Another defining characteristic of the macro optic is its close focusing distance. A close focusing distance requires the photographer to get very close to their subject in order to create a clear image through the camera sensor.
The typical macro lens has a focusing ratio of 1:1. However, optimal shooting distance varies dependent on what type of camera and lens you’re shooting with. True macro lenses will be primed - meaning that the focal length will be fixed without zoom capabilities.