This is from the Baltimore Catechism. I'm not Catholic but it's a pretty good definition.
Q. 162. What is a perfection?
A. A perfection is any good quality a thing should have. A thing is perfect when it has all the good qualities it should have.
Q. 163. What is God?
A. God is a spirit infinitely perfect.
Q. 164. What do we mean when we say God is "infinitely perfect"?
A. When we say God is "infinitely perfect" we mean there is no limit or bounds to His perfection; for He possesses all good qualities in the highest possible degree and He alone is "infinitely perfect."
Q. 165. Had God a beginning?
A. God had no beginning; He always was and He always will be.
Q. 166. Where is God?
A. God is everywhere.
Q. 167. How is God everywhere?
A. God is everywhere whole and entire as He is in any one place. This is true and we must believe it, though we cannot understand it.
Q. 168. If God is everywhere, why do we not see Him?
A. We do not see God, because He is a pure spirit and cannot be seen with bodily eyes.
Q. 169. Why do we call God a "pure spirit"?
A. We call God a pure spirit because He has no body. Our soul is a spirit, but not a "pure" spirit, because it was created for union with our body.
Q. 170. Why can we not see God with the eyes of our body?
A. We cannot see God with the eyes of our body because they are created to see only material things, and God is not material but spiritual.
Q. 171. Does God see us?
A. God sees us and watches over us.
Q. 172. Is it necessary for God to watch over us?
A. It is necessary for God to watch over us, for without His constant care we could not exist.
Q. 173. Does God know all things?
A. God knows all things, even our most secret thoughts, words, and actions.
Q. 174. Can God do all things?
A. God can do all things, and nothing is hard or impossible to Him.
Q. 175. When is a thing said to be "impossible"?
A. A thing is said to be "impossible" when it cannot be done. Many things that are impossible for creatures are possible for God.
Q. 176. Is God just, holy, and merciful?
A. God is all just, all holy, all merciful, as He is infinitely perfect.