Diatonic harmonica

Diatonic harmonica refers to various kinds of single-key harmonica:

Tremolo harmonica
A diatonic harmonica with two reeds per note.
In Asia, "diatonic" usually refers to this type.

Blues harp
A Richter-tuned ten-hole harmonica used in popular music.
In the United States and Europe, "diatonic" usually refers to this type.

Octave harmonica
A curved harmonica with Wiener Octave tuning