The name or term "Series" should not be confused with Model.
A Terminator Series is a line of nearly identical Terminators mass produced by Skynet. Units within a Series share common technological design, programming, material construction, and like chassis elements. Advances in Skynet's design theory intended to improve infiltration success rates have increased the available chassis sizing options within a Series, much in the way that like vehicles can be purchased with a wide variety of option packages. Additional features may be incorporated into subsequent production runs as situational demands merit. 
For series that support living tissue sheaths, Terminators with similar appearances are said to share the same model. Some models are known to support multiple series, such as the Model 101 used for both Series 800 and Series 850 Terminators.
It is not clearly established if the word "Series" or the series number should come first, such as "Series 800" or "800 series".
The term "Series" has not been applied to certain one-off Terminators. For example, the T-1000 was reportedly a prototype and was not called a "Series 1000" in Terminator 2: Judgment Day, though James Cameron did call it a "1000 Series" in his DVD commentary. The T-1000000 was also a unique Terminator, and was not part of a series.
↑Anti-tamper modifications were introduced to Series 888 Terminator CPUs to counter Resistance reprogramming efforts.
↑In the pilot of Terminator: The Sarah Connor Chronicles, John Connor asks Cameron, "What model are you? Are you new? You seem... different." He does not refer to her series, at that point or later. Series 888 Terminators on the show are often called "T-888", "Triple-Eights" or "T-Triple-Eights" but never "Series 888".