Batavianization or Batavianisation (see -ise vs -ize) also known as Dutchification and; historically, as Belgianization, is the spread of the Dutch language, people and/or culture either by force or assimilation. In Dutch it is known as vernederlandsing or neerlandisatie.
The term Batavianization, derives its name from the Batavi, a Germanic tribe living in the Netherlands, and long regarded as the mythical ancestors of the Dutch people. The term Dutchification is mostly used in English publications by Dutch authors, less so by native English-speakers. This likely has to do with the fact that Dutch isn't a Latin-derived term and hence sounds somewhat childish to some ears in combination with the (Latinate) suffix -fication. As for Belgianization, Belgae was the standard term in Latin to refer to the Dutch (compare the Leo Belgicus, United Belgian States and Belgic Confession for example) in the Late Middle Ages. After the establishment of Belgium, its use generally shifted to mean Belgicism.
In the Netherlands, most of the Batavianization was of a linguistic and, to a lesser extent cultural nature, and was focused on the Frisian region. Beginning at the end of the migration period, Dutch nobles sought to conquer the Frisian lands, in which they largely succeeded around the end of the High Middle Ages. The conquest was gradual and moved from the West to the East. By the time the Frisian heartland (the modern province of Friesland) was conquered, many Frisians formerly living in West Frisia had already fled and the region was subsequently colonized by Dutch settlers. In the remaining Frisian territory, a ruling Dutch upper class was instituted, the legacy of which can still be found in Stadsfries. The West Frisian language has since adopted large amounts of Dutch vocabulary, to such an extent that many objects or concepts originating after the Dutch conquest are nearly all calques or loanwords from Dutch.