aligner
noun
align·er
| \ ə-ˈlī-nər\
Medical Definition of aligner
: a clear, plastic orthodontic appliance that is molded to fit over the teeth and is used to correct their alignment
The aligners are made in a series: When the patient's teeth have been moved a tiny bit by one aligner, the next aligner goes in and moves them a bit further.— Fred Reed, The Washington Times, 15 May 2003