In linguistics usage, when distinguishing between southern and northern dialects in the Middle Dutch language, Diets meant southern language usage, whilst for northern features the term Duutsch was used. However, in other contexts these words were used as synonyms. They can stand equally well for Middle Dutch in general as for Germanic contrasted with Romance languages.
'Diets' is often confused with 'Duits', the modern Dutch word for German, and indeed in the Dutch language of the 16th and 17th centuries 'duits' and 'diets' were variant spellings of the same word, which usually is translated as Dutch (of the Dutch people) in modern Dutch.
The forms 'duits' and 'diets' originated as follows: the oldest Dutch (and German) form of the word is 'diut'; this led to the variants 'diet' and 'duut', and the adjectives 'diets' and 'duuts'. The latter form in Dutch changed to 'Duits' (Dutch vowel shift: the 'u' became the diphtong 'ui'). 'Duits' is still used in Dutch to refer to the German people and language, but the word 'diets', as well as the use of 'Duits' to refer to the Dutch people and language has fallen out of use.
Another reason 'Diets' is no longer used in modern Dutch is that the term was abused by 20th century fascists of the NSB and other nationalists, usually in the terms 'Diets', 'Nederdiets', or 'Nederduits' to refer to the shared heritage of the Dutch and German people.
"Wilhelmus van Nassauwe / Ben ik van Duitsen bloed"