Is 电视剧 equivalent to "TV series" or "TV drama"?
「電視劇」is equivalent to TV drama.「電視劇」does not tell you anything about the length of the series (if there even is a series) or the number of seasons in the series.


  If an English 电视剧 only has one season, can it be called series?


Yes. A TV series represents a connected group of TV shows with the same title/name. Series are only divided into seasons because of traditional American TV airing practice, where half the year is devoted to viewing and the other half is a re-run.

TV dramas are a product which responds to supply and demand; by nature, if a TV series is not performing by popularity metrics, then there will be no contract to make another season in the series. There are many single-season TV series out there (for various reasons, not only due to lack of popularity).

A TV drama series is called「連續劇」in Chinese. Dramas, in general, are just called「戲劇」, so you can get single "episode" (more like a mini-film) dramas called「單元劇」.