A Zionist is a follower of the political movement of Zionism — the effort to support and establish, or continue to support, the state of Israel. In parts of the Old Testament, or Torah, both Jerusalem and Israel are referred to as Zion. Zionism, the idea of providing an independent Jewish nation, began in the 19th century with the writings of journalists like Theodor Herzl. However, the idea that the state of Israel belongs to the Jewish people dates well before that. Judaism was born in the area now known as Israel and, as a result, Jews dating well before the common era (CE) claim Israel as their home.
Though Zionists have religious support for this contention, rooted primarily in the Torah, the Zionist movement was more political than religious. In fact, many of the people who fought for the establishment of a Jewish homeland were not particularly religious. Their primary goal was to end Jewish exile from their ancestral home, and so they identified more with the concept of a Jewish nation rather than the religion. The movement however, especially post Holocaust, caught fire and led to Israel's establishment in 1948. Early migrations to Palestine (the name of the area before 1948) were small compared to approximately 630,000 Jews who migrated en masse in 1947.
It's important to note that not every Jew is a Zionist. In fact, certain religious sects are strongly opposed the establishment of a Jewish state. Hasidic Jews, for example, tend to be critical of the movement, noting interpretations of the Torah that call for a Jewish state only after the arrival of the Messiah. An earlier return, it was argued, controverted God’s commands.
Opposition to Zionism also exists among Arabs and Muslims. In 1947, Arabs outnumbered Zionists two-to-one, but not all were opposed to the establishment of Israel. Still, Arabs and Muslims lived in the area for many centuries and the great and sudden influx of over 600,000 Jews in a relatively short period of time created great disruption to the way things had been pre-1947. As a result, Palestinian numbers declined. Many went to neighboring Jordan. Add to this dramatic demographic change, the fact that both Jews and Muslims claim Jerusalem as a site holy to their respective religions, and it is not difficult to see the beginnings of the Middle East conflict.
The term Zionist can refer to non-Jewish supporters of Israel as well. For instance, the governments of the United States and United Kingdom have supported Israel. Many people, especially after the Holocaust, felt that the establishment of a Jewish state was a just and right cause. After witnessing the horrors and mass executions, many sympathized with the cause for a Jewish homeland.
Much goes back to the way the Jews in exile had been moved from place to place for over two millenia, often against their will. They encountered discrimination in virtually every place they settled, including Egypt, Greece, medieval Europe, and 20th century Russia and Eastern Europe, where just being Jewish was cause for discrimination.
Yet being Zionist remains an contentious issue, particularly in the Arab and Muslim world, where it is generally viewed that the Jews had no right to take over an area where others were already living. Christianity also has a claim to Jerusalem — the birth place of the religion. Still, most Christian sects are Zionist and support Israel, asserting that the Jews had the right their ancestral homeland and to establish their own nation. Moreover, many Zionists do not contest Islam's and Christianity's right to claim Jerusalem and Israel as central to their respective religions, but Zionists generally feel their religion has a higher claim, because Judaism is the oldest of the three religions.