Thorough Bass; Figured Bass; Basso Continuo

Note:  "thorough" (old spelling for "through") means the same as "continuo", i.e., continuing throughout the piece

A method of indicating an accompanying part  by the bass notes only, together with figures designating the chief intervals and chords to be played above the bass notes.  This stenographic system was universally used in the baroque period (1600-1750).

The thoroughbass practice of the Baroque grew out of the improvisation techniques of the 16th century.  About 1600, motets were occasionally accompanied on the organ by a "bassus pro organo,"  a separate bass part from which the organist played the harmonies.

"Basso continuo" refers to the instruments used to play the thoroughbass (figured bass) part -- usually, in the Baroque period, an organ or harpsichord and a cello or "viola da gamba".