A single thing, and the beginning of counting. Also wholeness — the same word for a unit and for being undivided.
Old English ān (one), from Proto-Germanic *ainaz, from Proto-Indo-European *oi-no- (single, unique); the same root that gives Latin unus and Greek oinos (the one on a die).
The first count and the mark of being undivided. Where the house needs to say a single thing, or a thing that cannot be split, this is the word for both.