An extreme close-up lens. This may be a complete camera lens, or a lens attachment which screws into the filter ring or bayonet mount of the main camera lens.
A macro lens is usually associated with a ratio such as 1:1 or 1:2 etc. The first number is the relative size of the image on the film, the second number is the relative size of the subject, so read a:b as "a on the film, b in real life".
SLR macro lenses can be focused from infinity down to 1:1 (life-size on the film), or 1:2 (half life-size on the film).
Nikon refers to its SLR macro lenses as `micro' lenses, e.g. `60mm AF Micro-Nikkor'. A removable lens can be converted into a macro lens by the addition of an extension tube.