Zionism doesn't have a single meaning, and probably never did.
At one time, it was the movement to persuade the British to create Israel out of the Palestinian Mandate, which Britain occupied from and after WWI, and which comprised FAR more than just what is Israel and Palestine today.
That being accomplished, the original Zionist movement is no more.
Today the term's meaning depends on who uses it. To some it's anyone that wants to see Israel continue to exist. To others it's the secret movement that wants Israel to control the entire world and exterminate all Muslims. The first category is easy to find, they're all over Israel itself, and in many other parts of the world. The second is a little harder to get ahold of, there doesn't seem to be any known leader or even a post office box to write to them, and they don't seem to have a website. People like Achmidenajad seem to know who they are, though.
There are a lot of Christian Zionists, too. They believe that the Second Coming depends on the existence of Israel, so they support Israel more fervently than most US Jews even do.
I guess the bottom line of what it means to be a Zionist is one that goes beyond defining Israel as a sovereign nation, and including any kind of religious significance to Israel. That would include Israelis, Arabs, Muslims, Jews, Christians, and yes, that makes Mahmoud Achmidenajad a Zionist.
Israel is a sovereign nation. That's it. It happens to be largely Jewish, just like the US happens to be largely Christian or Agnostic, depending on who you ask.