Were the Aztecs a tribe?

The Aztec people were a certain ethnic groups of central Mexico.

There was no such thing as "Aztecs", that is a modern word coined several centuries afterwards. The people were called the Mexica. They formed numerous city-states and had formed an alliance of the three most powerful, and this alliance is what is usually dubbed the so called "Aztec Empire".

As with most city states and kingdoms, their roots would have been in a tribe of people.

The Aztecs were a tribe which formed a city-state which then became an empire by it's conquests of other tribes.

The Aztecs, or Mexicas, later Republic of Mexico was named, were a Mesoamerican people of central Mexico in the 14th, 15th and 16th centuries. They were a civilization with a rich mythology and cultural heritage and an Empire..........

http://library.thinkquest.org/27981/

They are related ethnic groups, in the modern period they are known as the Nahua peoples. Over 1 million Mexicans speak the Aztec language Nahuatl