Judicial "Activism" Isn't the Issue:
Jeff Rowes of the Institute for Justice has an important op-ed in today's Wall Street Journal: Judicial 'Activism' Isn't the Issue: Liberals and conservatives both show too much deference to Congress. Here is how it begins:
The growing dispute between conservatives and liberals over the Supreme Court nomination of Sonia Sotomayor obscures a more troubling point of agreement: The government should almost always win.

Many conservatives who think of themselves as proponents of limited government would be surprised to discover that conservative judges begin their constitutional analyses in almost every context by placing a thumb firmly on the government side of the scale. It's called "judicial deference." Many liberals, who take pride in being "empathetic," would be surprised to learn that liberal judges also subscribe to judicial deference.

The practical result is that judges of both persuasions almost never enforce any constitutional limit on the power of government to regulate property and the economy. Given that the vast majority of law concerns these two areas, the real crisis in constitutional law is not judicial "activism" but judicial passivism.

It all began in the late 1930s, when the Supreme Court opened the floodgates for New Deal economic regulation. In essence, conservatives have adopted the big-government agenda of that era. The liberal-conservative consensus explains why nomination fights focus on a few "culture war" issues such as gay marriage or guns. Liberals and conservatives squabble over these esoteric questions because there is such harmonious accord on everything else.
Then there is this:

The Constitution's framers understood that legislatures are as much nests of vice as of virtue. That is why they went to such lengths to define the limits of government, set forth our rights broadly, and create an independent, co-equal branch of government to protect those rights.
Read the whole thing.

The only thing I would add is that the epithet "judicial activism" could be used to describe departing from the requirements of the Constitution, whether to uphold or strike down legislation. On the very rare occasions when I use the term, I am careful to define it in this way.