Polyrhythms are rhythmical patterns that do not follow the standard subdivisions of 2, like 1/2, 1/4, etc. This includes 1/3, 1/5, 1/7, and all others. 1/3 is specifically called "triples" as it's more common and isn't as exotic as the rest.
Polyrhythms are usually denoted in scores as a X:Y value. The real duration of the polyrhythm phrase (the collection of notes that are grouped together with the symbol) depends on the Y value, and the subdivision that the polyrhythm involves. The subdivision is implied and not mentioned in the X:Y notation. We assume that the subdivision is a value Z, which we determine from the subdivisions of the notes within the phrase. Usually (but not always) the highest subdivision we encounter is the subdivision of the polyrhythm. For example, if we have a phrase that has 1/8 and 1/16 notes, then the subdivision of the polyrhythm phrase is 1/16, unless the numbers don't add up.