Tefillin “phylacteries”, are composed of two teffilos, the teffilo “phylactery” shel yad and the teffilo shel rosh, together, they are a pair of teffillin.
The Dictionary of Jewish usage: a guide to the use of Jewish terms says: “Although “tefillin” is technically the plural form (the singular being “tefillah”), it is loosely used as a singular as well.”
The Merriam-Webster Dictionary also writes: Tefillin is plural in form but sometimes singular in construction.
As a concept, either singularly or plural, it is called tefillin.