The Collaborative International Dictionary
Theorbo \The*or"bo\, n. [F. th['e]orbe, t['e]orbe, formerly tuorbe, tiorbe, It. tiorba.] (Mus.) An instrument made like large lute, but having two necks, with two sets of pegs, the lower set holding the strings governed by frets, while to the upper set were attached the long bass strings used as open notes.
Note: A larger form of theorbo was also called the archlute, and was used chiefly, if not only, as an accompaniment to the voice. Both have long fallen into disuse.
Wiktionary
n. (context musical instruments English) A stringed musical instrument rather like a large lute
Wikipedia
The archlute (Spanish archilaúd, Italian arciliuto, German Erzlaute, Russian Архилютня) is a European plucked string instrument developed around 1600 as a compromise between the very large theorbo, the size and re-entrant tuning of which made for difficulties in the performance of solo music, and the Renaissance tenor lute, which lacked the bass range of the theorbo. Essentially a tenor lute with the theorbo's neck-extension, the archlute lacks the power in the tenor and the bass that the theorbo's large body and typically greater string length provide.
The main differences between the archlute and the "baroque" lute of northern Europe are that the baroque lute has 11 to 13 courses, while the archlute typically has 14, and the tuning of the first six courses of the baroque lute outlines a d-minor chord, while the archlute preserves the tuning of the Renaissance lute, with perfect fourths surrounding a third in the middle for the first six. The archlute was often used as a solo instrument for the first three-quarters of the 17th century, but is rarely mentioned as a continuo instrument in this period, the theorbo being the lute class instrument with this role.
As continuo bass lines were composed both faster in motion and higher in tessitura towards the end of the 17th century, the archlute began to eclipse the theorbo as the main plucked string continuo instrument. The theorbo lacked the higher notes of the bass lines and the increasing practise of doubling the continuo part with a bowed bass ( cello or viol) made the archlute's lack of power in the tenor and bass a less important shortcoming.
The theorbo had been commonly used as the melodic bass instrument in trio sonatas from the beginning of the Baroque and the archlute took over that function too, with the most famous example being Corelli's Opus 1 and 3 trio sonatas which have partbooks for 1st and 2nd violin, 'violone o arciliuto' and a continuo part for organ, a simplified version of the 'violone o arciliuto' book. The violone o arciliuto book has just as many figures to tell the player what chords to play as the organ partbook, which suggests the archlute player would be adding chords above the bass where possible.
The archlute was used in Handel's operas and like repertoire; Giulio Cesare (1724) has continuo parts labelled both arciliuto and tiorba. Perhaps one player would play both instruments.
Music for solo archlute is usually notated in tablature.
Usage examples of "archlute".
The violin and the other members of its family had taken their places somewhat as we now have them, but the number of basses and tenors was much less than at present, their place being filled by the archlute and the harpsichord.
One Archlute in South Kensington Museum has as many as 24, eleven of which are duplications.