A copyist is a person who makes copies.
The term is sometimes used for artists who make copies of other artists' paintings. However, the modern use of the term is almost entirely confined to music copyists, who are employed by the music industry to produce neat copies from a composer or arranger's manuscript.
Until the 1990s, most copyists worked by hand to write out scores and individual instrumental parts neatly, using a calligraphy pen, manuscript paper, and often a ruler. Producing parts for an entire orchestra from a full score was a huge task. In the 1990s, copyists began using scorewriters - computer programs which are the music notation equivalent of a word processor. (Such programs include Sibelius, Finale, MuseScore or GNU LilyPond). Scorewriters allow the composer or songwriter to "enter" the melodies, rhythms and lyrics to their compositions into the computer using a mouse or by playing the notes on a MIDI-equipped keyboard. Once a composition is fully entered into a scorewriting program, the computer can be instructed to print out the parts for all of the different instruments.
Both handwritten and computer-based copying require significant understanding of musical notation, music theory, the musical styles and conventions of different styles of music (e.g., regarding appropriate ornamentation, harmony rules pertaining to accidentals, etc.), and strong attention to detail and past conventions. Ludwig van Beethoven had a contentious relationship with his copyists, who often made mistakes that remained uncorrected until the advent of Urtext editions; some musicologists have devoted a lot of effort to identifying Beethoven's copyists.