The word monad comes from the Greek monas — "unit," "alone," "singular" — derived from monos, "single" or "alone." In its most basic mathematical sense, the monad is simply the number one: the unit from which all other numbers are generated by addition. But in Pythagorean and later philosophical traditions, the number one is never simply a quantity — it is the principle of unity itself, the formal cause of all ordered multiplicity, the point from which all dimensionality proceeds without itself having dimension.
This is the first and most fundamental property of the Monad across all traditions: it is indivisible. You cannot cut a unit without creating fractions — two halves, each of which is a new unit at a smaller scale. The Monad is what cannot be further reduced: the stopping point of analysis, the irreducible remainder. In mathematics, 1 is the multiplicative identity — multiply anything by 1 and it remains itself. This is the Monad's philosophical character: it preserves without adding, sustains without changing, unifies without merging.
The Monad's second universal property is its generativity: from the One comes the Many. This appears differently in each tradition — in Pythagorean mathematics, all numbers proceed from the monad through its self-relationship; in Neoplatonism, all being emanates from the One through a series of necessary overflows; in Gnosticism, all aeons (divine beings) proceed from the supreme Monad through a process of thought and self-expression. But the structure is the same: one irreducible source, from which multiplicity unfolds without the source itself being diminished.