In grammar, number refers to how many of something one is talking about, both for people and for things. The three most common numbers are singular, dual and plural. In both Indo-European (see the related reference page) and Semitic languages where it occurs, dual is reflected by inflections for both nouns and verbs and their modifiers, as are singular and plural.
As one should expect, singular refers to only one of a given noun, pronoun or in agreement adjective. Yet singular is not the same as the lack of a number. Thus for example, Chinese in its various forms does not use the singular; rather Chinese (whether Mandarin, Cantonese or some other language so designated) has no number whatsoever– whether singular, dual or plural.
Again, as one would expect dual refers to exactly two. The oldest Indo-European languages (such as Sanskrit and Hittite) include a robust dual number. Classical Greek includes a dual in some instances in Homer, but for the most part the dual number had vanished from Greek by the Classical period. Latin shows no evidence of a dual, even residually.
Among Semitic languages, the dual is more common. Hebrew includes a dual form residually and Arabic more robustly, for example.
Accordingly, is the last of the three most common numbers. When the dual occurs robustly, it denotes three or more, but when the dual is residual or non-existent, it denotes two or more.