When is a Tercile not a Tercile? I have data being grouped into 3 ranges: 80-89%; 90-109% 110-120%
Can I call these 3 Terciles?  I believe a tercile is the data split into 3 equal sized groups - is there another term I can use instead of tercile (for uneven groups)?
 A: From my understanding, and as you pointed out, terciles are three intervals containing 1/3 of the observations each. My suggestion to you would thus be to instead use another word to describe your intervals with, in order to avoid misunderstandings. Perhaps "partitions", "discretisations", or "chosen intervals" would suffice? 
A: A tercile contains one-third of the data with the cuts being lower third, middle third and upper third. A tercile is an element of that particular subset of data divided into three groups that is either lower, middle or upper thirds. 
Generally, if the number of data entries is not a multiple of three, interpolation is used to calculate the tercile parameters. 
The other term the OP wants is merely, "data divided into three groups," which although not implying interpolation at face value, does not exclude it either.
A: A term which I have seen used for groups formed by cutting at the tertiles is "tertile categories". The advantage of this over saying thirds is that it makes precise how the categories were arrived at. Of course quintile categories and so on are also in use.
