Wikipedia
Sight-reading, also called a prima vista (Italian meaning "at first sight"), is the reading and performing of a piece of music or song in music notation that the performer has not seen before. Sight-singing is used to describe a singer who is sight-reading. Both activities require the musician to play or sing the notated rhythms and pitches. Many believe that individual–sight-singing is the more challenging of the two, because the musicians do not have any keys, frets or valves (on keyboard instruments, guitars, and valved brass instruments, respectively) to help them obtain the correct pitches. However, difficulty is related both to the instrument and the difficulty of the piece itself. For instance, sight-reading on a polyphonic instrument, such as the classical guitar, can be more challenging due to the fact that the guitarist has to read polyrhythms and polyphonic passages that can often be played in different positions using different frets.