The nasal infix is a reconstructed nasal consonant or syllable *⟨n(é)⟩ that was inserted (infixed) into the stem or root of a word in the Proto-Indo-European language. It has reflexes in several ancient and modern Indo-European languages. It is one of the affixes that marks the present tense.
In the Proto-Indo-European language (PIE), the nasal infix *⟨n(é)⟩ is one of several means to form the athematic present tense. It is inserted immediately before the last consonant of the zero-grade root.
The infix appeared as *⟨né⟩ in the forms where a full-grade stem would be expected, and as *⟨n⟩ in forms where zero-grade would be expected. For example, the PIE root *weik- "to win" would yield a nasal-infixed present stem *wi⟨né⟩k- ~ *wi⟨n⟩k-.
These presents are called nasal infix presents or simply nasal presents and are typically active transitive verbs, often with durative aspect.
Since the linguistic ancestor of PIE is not known, there can only be speculations about the origins of the nasal infix. It has been suggested that it arose from a suffix (also related to *-neH- and *-neu-) which underwent metathesis.