The term "activist" is often interpreted as "legislating from the bench" where the judiciary usurps the role of the legislature. Some people actually want that. Other people don't want that.
Refusing to solve a problem and instead requiring Congress to clarify the law is another judicial philosophy. Is that being an activist?
Deciding that the federal government has no authority and that state authority or individual rights are more paramount is also a course of action that some people agree with and some people don't. Supporters probably don't call that "activism" but detractors might.
So I think the term is mainly used to slur your political opponent as opposed to being a succinct term for some particular judicial philosophy.