In layman’s terms the inner tube of a tire or if you prefer, a doughnut or a vadai. Technically, it is the surface of revolution of a circle about a coplanar axis outside itself (to put that in context, a sphere is a surface of revolution of a circle about one of its diameters, i.e., a coplanar axis through its center).
Why am I talking about it?
I always thought that a torus is a strange object, almost like a mobius strip or something. Mathematically it is a weird compact manifold. And so I imagined that a torus is as likely to occur in nature as a cube. But then, I found out today that certain dumbbell shaped surfactants self assemble into nanoscale tori ! Further digging and googling told me something I should have known, that people who make Carbon nanotubes have actually seen Carbon nanorings that form at the same time, that are essentially tori.
What does this mean?
So, the above facts tell us that self assembly can lead to tori. If it is easy to form and stays stable, then more likely than not, we will find it in nature. And indeed we do. They call it (rather unimaginatively) the torovirus.
Goes to show that one has to modify and augment one’s intuition with each new fact learnt!
 A Primer on self assembly of surfactants available here.