It's the masculine form of the Latin word "unus", which can be used as:
* Adjective: Meaning "one" or "single"
* Noun: Meaning "one" (as in, "the one")
* Pronoun: Meaning "one" (as in, "one should always strive to be kind")
Examples:
* Unus homo: One man
* Unus annus: One year
* Unus et alter: One and another
* In unam rem: Into one thing
Note: "Unus" is often used in conjunction with other Latin words, such as "duo" (two), "tres" (three), and "quatuor" (four), to form compound numbers.