Noun
A telescope arranged and used to determine errors of collimation, both vertical and horizontal.
A tube having a convex lens at one end and at the other a small opening or slit which is at the principal focus of the lens, used for producing a beam of parallel rays; also, a lens so used.
Source: Webster's dictionaryA wheel type radiotherapy device which has a long collimator to focus the radiation into a narrow beam. Source: Internet
Collimation is done either with a collimator (basically, a long tube) or with a clever arrangement of gently curved mirrors. Source: Internet
In the isolated direction the beam is split, and then diverged, so it does not focus at the collimator. Source: Internet