As physicists have proven, and ancient yogic texts have let us know for thousands of years, there is nothing in this reality that is not spirit, or as one might call it, "energy". Atoms, electrons, and neutrons, the building blocks of our world, have been examined thoroughly, and there is nothing solid there. There is only vibrating energy. Those atoms, electrons, and neutrons are the everything of everything: they make up the food we eat, the air we breathe, the cars we drive, the people we know. So if all of reality is only energy -- that table, this chair, your hair, your coffee--and not solid at all, why does it seem to be so? And here is the beginning of the great mystery.
The term "spiritual" relates to identification with spirit, which some may refer to as inner power, others may refer to as that aspect of us that differentiates our alive bodies from corpses, or even as the part of us that is not as temporary as our homes, cars, toys, and bodies. The part of us that is permanent. It is that very vibration that physicists have detected as the building blocks of our world--it is pure energy. (http://en.wikipedia.org/wiki/Quantum_mechanics)
The spiritual world is far more vast than that which appears as our physical world, and it is even difficult (if not impossible) to comprehend with our feeble minds. In yoga, it is often described in this way: the spiritual aspect of the world is an elephant. We are all blind people given one square foot of the elephant to explore with our fingertips. It is from this one square foot that we must deduce what this elephant (spiritual world) is. And it is no wonder that we disagree: some of us might be discovering a foot, while others explore a tusk, and still others claim the spiritual world is the elephant's belly. We are all correct, but not in describing the elephant. We are correct only in describing the limits of our minds.