Is "series" Plural or Singular?

Such expressions as

Drama series

and

TV series

are plural or singular?

E.g. I like to watch drama/TV series or I like to watch a drama/TV series?

Answer

"Series" can be singular or plural depending on context. Both of your sentences are therefore correct but different.

I like to watch TV series.

uses series as a plural and means you like to watch a number of different TV shows.

I like to watch a TV series.

uses series as singular and means you like to watch one TV show.