A minyan is the threshold that turns some prayers from private devotion into public worship.
A minyan is the quorum for communal prayer
Britannica defines a minyan as the minimum number required to constitute a representative Jewish community for liturgical purposes. In classical usage that number is ten adult males, though Conservative and Reform communities often count women as well.
That difference matters because a minyan is not just attendance. It is what lets a congregation pray as a public body.
Some prayers require a minyan and some do not
Britannica notes that without a minyan there is no public Torah reading and prayers such as Kaddish and Kedusha are omitted.
This is the practical point. Jews can pray alone, but not every form of Jewish prayer is meant to be solitary.
The concept reflects a communal religion
The minyan matters because Judaism is not built around isolated spirituality. Covenant, memory, and worship are carried by a people, not only by individuals.
That is why the absence of a minyan changes the service. The issue is not whether private prayer counts. It does. The issue is whether the community is present as a community.
Why it still matters
The minyan still matters because it teaches dependence. Jewish worship sometimes requires other Jews in the room. That can be inconvenient, but it is also the point.
The shortest accurate answer
A minyan is the quorum required for certain forms of Jewish public prayer, marking the difference between private devotion and communal worship.