There are many definitions of Zionism, some right, some wrong, some partially right, some partially wrong. A good, concise definition is the philosophy that Jews, the Children of Israel should return to the land of Israel, live there, and secure and defend their lives there.
Anti-Zionists on the internet like to wave around groups like the “True Torah Jews” and the “Neturei Karta” as examples of Jewish anti-Zionists. While it is true that the most religious Jewish groups originally were opposed to modern Zionism, it was mainly because semi-secular groups were leading it, and not them. Today, things have changed, and most very religious Jews support modern Zionism to at least some extent. However, opposition still exists, both in Israel and abroad.