Everything that is that is not God. The physical universe. Humans. Spiritual beings (angels and demons etc).
CCC 325: The Apostles' Creed professes that God is "creator of heaven and earth". the Nicene Creed makes it explicit that this profession includes "all that is, seen and unseen".
CCC 326: The Scriptural expression "heaven and earth" means all that exists, creation in its entirety. It also indicates the bond, deep within creation, that both unites heaven and earth and distinguishes the one from the other: "the earth" is the world of men, while "heaven" or "the heavens" can designate both the firmament and God's own "place" - "our Father in heaven" and consequently the "heaven" too which is eschatological glory. Finally, "heaven" refers to the saints and the "place" of the spiritual creatures, the angels, who surround God.
CCC 338: Nothing exists that does not owe its existence to God the Creator. the world began when God's word drew it out of nothingness; all existent beings, all of nature, and all human history are rooted in this primordial event, the very genesis by which the world was constituted and time begun.
In this all the branches of Trinitarian Christianity are agreed.