A contrabass guitar is a low-register plucked string instrument in the guitar family with five or six strings. It is often called, simply, a six string bass guitar. The five string bass guitar is rarely called a contrabass guitar, even though it typically has the same lowest note.
A “classical” contrabass guitar is tuned EADGBE, like the classical guitar, but one octave lower. It is popular in Fado bands in Portugal and South America. In this sense, it may actually be considered as an acoustic bass guitar, for it shares the same low-end range. Although called a contrabass guitar, the fact that it is tuned only an octave lower than a normal guitar makes it—in modern terminology—more an acoustic bass guitar than a contrabass instrument. Its strings are much thinner than a conventional acoustic bass guitar, so it lacks the “thick” tone of those instruments.
Prelude to Cello Suite #1, J. S. Bach, contrabass classical guitar