Acts 2:1-6 1 When the Day of Pentecost had fully come, they were all with one accord in one place. 2 And suddenly there came a sound from heaven, as of a rushing mighty wind, and it filled the whole house where they were sitting. 3 Then there appeared to them divided tongues, as of fire, and [one] sat upon each of them. 4 And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance. 5 And there were dwelling in Jerusalem Jews, devout men, from every nation under heaven. 6 And when this sound occurred, the multitude came together, and were confused, because everyone heard them speak in his own language.
In this context, "tongues" refers to languages. According to Strong's Greek dictionary (entry #1100), the word glossa implies a language, specifically one that the speaker did not naturally acquire.
When the Holy Spirit descended upon the apostles and everyone present, the individuals themselves were not the originators of the speech. Instead, the Holy Spirit spoke through them.