A zero-marking language is one where there tend to be no grammatical marks on either the dependents or modifiers or the heads or nuclei showing the relationship between different constituents of a phrase.
Pervasive zero marking is very rare, but instances of zero marking in various forms do occur in quite a number of languages. In many East and Southeast Asian languages, such as Thai and Vietnamese, the head verb and its dependents are not marked for any arguments or for the nouns' roles in the sentence.
Some languages use a similar process, called "juxtaposition" in linguistic jargon, to indicate possessive relationships. The rarity of pervasive zero marking is because languages with juxtaposition have very much higher levels of inflection than languages with zero marking in noun phrases, so that the two almost never overlap.
Zero-marking, where it does occur, tends to show a strong relationship with word order. Languages where zero-marking is widespread are almost all subject–verb–object. This is perhaps because verb-medial order allows two or more nouns to be recognised as such much more easily than either subject–object–verb or verb–subject–object order where two nouns might be adjacent and therefore their role in a sentence possibly confused. It has been suggested that verb-final languages may be likely to develop verb-medial order if marking on nouns is lost.