The Greek word for Tongue is glossa. This word is defined by Thayer's Greek Lexicon as: a tongue, i.e. the language used by a particular people in distinction from that of other nations: Acts 2:11. Throughout the New Testament the word tongue is clearly a language or dialect used by a particular people distinct from that of other nations. The concept that a tongue is a language unknown to mankind may come in part because of the way the King James Version translates certain passages. The King James Version added the word unknown before the word tongue in their translation. The word unknown is never found before the word tongue in any of the original manuscripts. Understand that the translators of King James Version never intended to show or imply that it was a part of the original writing of Paul. They KJV translators often inserted a word that they felt would help the reader understand what the original text was saying. Because of the great task of translating one language to another (as here they were translating Greek into English) sometimes it is necessary to do this. But when they did it, they put the inserted word in italics. So it says in the text of the KJV, “…unknown tongues…” This lets the reader know that the word unknown is not part of the original manuscripts. And in this case, this probably causes a lot of confusion. If the word “unknown” was used to say that a tongue was a real human language unknown to the person speaking the tongue, then this is a valid insertion of the word. But if we take the word “unknown” to mean a language unknown to mankind, such as an angelic language, then this word is wrong. The best thing to do here is read the Bible as written in the original manuscripts and interpreted properly by most modern versions, which did not insert the word 'unknown'.
One of the best examples of the definition of a tongue is found in Acts 2 where the apostles began to speak in languages that they were not able to speak before. We see here that the apostles spoke specific languages that already existed. Speaking in tongues was nothing more than the supernatural ability to speak in a real human language.
Acts 2:6-12 When they heard this sound, a crowd came together in bewilderment, because each one heard them speaking in his own language. Utterly amazed, they asked: "Are not all these men who are speaking Galileans? Then how is it that each of us hears them in his own native language? Parthians, Medes and Elamites; residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the parts of Libya near Cyrene; visitors from Rome (both Jews and converts to Judaism); Cretans and Arabs-we hear them declaring the wonders of God in our own tongues!" NIV
Notice here that these apostles were speaking in known languages and those that heard them speak in tongues heard them speaking in their own language. So we can read clearly that this gift of tongues was not some unknown language. In fact the Bible lists the specific native languages that were spoken.
Tongues in the Bible were real human languages that were known to man (They were never Angelic Tongues. See that page for further details). The gift of tongues was the ability to speak in a foreign language without having to study and learn that specific language. The tongue was known to someone, just not the speaker. This gift is no longer given today.
For an in depth study on this topic please read our page titled "Spiritual Gifts."