(adjective) - Of or pertaining to the Netherlands. In art history, Netherlandish refers to art created in present day Belgium and The Netherlands (also known as "Holland") before 1830.
The word Netherlands really just means "low countries." Because, during the Northern Renaissance, neither Holland nor Belgium was an independent entity - and we always seem to need classifications in art history - we've employed the all-encompassing term "Netherlandish."
Starting around the 16th century, it became common to call artists of the southern provinces (which were Catholic, and controlled by Spain) "Flemish," while artists from the Protestant northern provinces were known as "Dutch." The use of the term Netherlandish (for either) persisted for some three-hundred years afterwards, however.