as a noun, 'series' refers to a number of similar or related events, objects, or people coming one after another. It can also refer to a set of television or radio broadcasts on the same subject or using the same characters but in different situations.
'series' is both singular and plural, which can sometimes lead to confusion. When referring to one set or sequence, you use 'series' in the singular form, and when referring to multiple sets or sequences, you still use 'series' but the context or accompanying verbs will indicate the plurality.
I watched the entire series in one weekend.
There are several series of tests you must undergo.
The Harry Potter series is popular worldwide.
remember that 'series' remains 'series' in both singular and plural forms. The context will help determine its number. Be cautious with verb agreement. For example, 'The series is interesting' (singular) vs. 'Both series are interesting' (plural).