Being Politically Correct: Are Ethnic Groups called Tribes or Nations?

Growing up in Nigeria, one of the first things you learn in your social studies class is that Nigeria has three major ethnic groups: Igbo, Yoruba and Hausa.  For whatever reason, the teachers never mentioned that we had over 200 ethnic groups.  I found out about that when I came here.

While I was in Nigeria though, it was perfectly acceptable and common to refer to ethnic groups as "tribes."  I never heard anyone say it was offensive or even insinuate that it could be offensive to say a person was from a particular tribe.  That is, until my college days here in the United States.