Found 50 or so times in the New Testament, the Greek noun “glossa” meant “tongue” or “language.” In places like Mk. 7:33, 35; Lk. 1:64; 1 Jn. 3:18, this word is used to describe a literal tongue. This word is also used to describe the “tongues” which appeared on the Day of Pentecost (Acts 2:3). The apostles began to “speak with other tongues” (Acts 2:4), and these tongues are described as human “languages” (Acts 2:8).
Some have expressed a great interest in “unknown” tongues (1 Cor. 14:13-14, KJV), but “unknown” simply means a language which is not understood by a hearer. If someone who speaks only English hears someone speaking Spanish, the English listener is hearing a real, human language, but this “tongue” is “unknown” to him.
For a fuller study of tongue speaking see this file about tongue speaking from our downloadable chapter by chapter First Corinthians commentary.