A term from Sanskrit phonetics, used in English. In the Devanagari script it is a dot above a syllable on the right, and indicates the vowel is nasalized. In romanization this is indicated by an m with a dot under or over it; on the Web we write either m. or (in The Harvard-Kyoto system of Transliteration) M, to distinguish it from the ordinary consonant m.
In sandhi, the change of consonants depending on what sound follows, the anusvara becomes a full m before vowels, and finally. For example, the neuter nominative/accusative singular ending is am or (before consonants) aM, as in nagaraM 'city' corresponding to Latin -um and Greek -on. These come from the ending *-om in their common ancestor Proto-Indo-European, and in all three the same ending is also used for the masculine singular accusative.
The second A of the name is long (indicated in print with a macron), so it may be seen on the Web as anusvaara or anusvAra.