poly - many
dáktulos - finger
Polydactyla: multiple finger (double notes) passages such as (possibly alternating) thirds, fourths, fifths, or sixths.
Pieces based on polydactyla: Chopin's Op. 25 Nr. 6, Liszt's Feux Follets, Schumann's Toccata.
In polydactyla passages such as thirds scales it is very helpful to practice (and play) the upper (leading) voice legato and the lower voice non legato.