Heaven is where God is.
Also, from the Baltimore Catechism #3:
Q. 1395. What is Heaven?
A. Heaven is the state of everlasting life in which we see God face to face, are made like unto Him in glory, and enjoy eternal happiness.
Q. 1396. In what does the happiness in heaven consist?
A. The happiness in heaven consists in seeing the beauty of God, in knowing Him as He is, and in having every desire fully satisfied.