Being a single entity, unit, object, or living being. Characterized by unity; undivided: They spoke with one voice. Of the same kind or quality: two animals of one species. Forming a single entity of two or more components: three chemicals combining into one solution. Being a single member or element of a group, category, or kind: I'm just one player on the team. Being a single thing in contrast with or relation to another or others of its kind: One day is just like the next. Occurring or existing as something indefinite, as in time or position: He will come one day. Occurring or existing as something particular but unspecified, as in time past: late one evening. Informal Used as an intensive: That is one fine dog. Being the only individual of a specified or implied kind: the one person I could marry; the one horse that can win this race. The cardinal number, represented by the symbol 1, designating the first such unit in a series. A single person or thing; a unit: This is the one I like best. A one-dollar bill. An indefinitely specified individual: She visited one of her cousins. An unspecified individual; anyone: "The older one grows the more one likes indecency\u201d ( Virginia Woolf). at one In accord or unity. one and all Everyone. one by one Individually in succession.