I found a great (but LONG) definition of "refomed" from a Professor Byron Curtis, a professor at Geneva College. He breaks the definition into four parts. To be Reformed is:
To confess the consensus of the five first centuries of the church:
Classic theism: One omnipotent, benevolent God, distinct from creation.
Nicene and Chalcedonian Trinitarianism: one God in three eternally existent persons, equal in power and glory.
Christ, the God-Man, the one mediator between God & the human race, incarnate, crucified, resurrected, ascended, & coming again.
Humanity created in the image of God, yet tragically fallen & profoundly in need of restoration to God through Christ.
The Visible Church: the community of the redeemed, indwelt y the Holy Spirit; the mystical body of Christ on earth.
The one, holy, catholic, and apostolic church.
The Sacraments: visible signs and seals of the grace of God, ministering Christ’s love to us in our deep need.
The Christian life: characterized by the prime theological virtues of faith, hope, and love.
It would be correct to say that this is a statement of the Protestant faith more than it is a statement of the Reformed faith. From this list we see that Reformed Christians adhere to all the foundational beliefs taught in the Bible. These beliefs were the foundation of the early church and are based on the teachings of the Bible as interpreted by the apostles and early church fathers. Many of these beliefs were changed or lost as the Catholic Church grew in power and authority from the fifth century onwards. Throughout history there were pockets of non-Catholic believers who held to many or all of these points of doctrine, but they were largely lost until the time of the Reformation.
To confess the four solas:
The authority of Scripture: sola scriptura (Scripture alone)
the basis of salvation: Sola Gratia (Grace alone)
the means of salvation: Sola Fide (Faith alone)
the merit of salvation: Solus Christus (Christ alone)
Again, these form the basis for Protestantism as much as they do for the Reformed tradition. These are the principles that drove the Protestant Reformation of the 16th century and separated it from the Roman Catholic Church. These four points of doctrine are based entirely on the Bible and were the theological driving force behind the newly formed Protestant movement.
To confess the distinctives of the Reformed faith:
In salvation: monergism not synergism. God alone saves. Such monergism implies T.U.L.I.P., the Five Points of Calvinism from the Synod of Dordt:
T = Total Depravity U = Unconditional Election L = Limited Atonement, or, better, Particular Redemption I = Irresistible Grace P = perseverance and Preservation of the Saints
These five distinct points of doctrine are also known as the five points of Calvinism as they were first articulated by John Calvin after the Reformation was in full-swing. They are based entirely on the Bible. When people speak of being Reformed these five points of doctrine are most often what they are referring to. Most evangelical (non-Reformed) churches do not hold to all of these points. Some hold to two or three (and occasionally even four), but most reject them in favor of Arminian theology. For a more in-depth look at what constitutes Reformed vs Arminian theology, see my four part article which begins here.
Other Reformed Distinctives:
Professor Curtis goes on to list other points of doctrine he believes are Reformed distinctives. They include: The Regulative Principle of Worship, Covenant theology and Life is religion (Christians have neither jobs nor careers; they have vocations (callings)). I would not consider adherence to these principles necessary to consider oneself Reformed and I suspect the majority of Reformed Christians would agree with me.
Finally: in everything, Soli Deo Gloria - to God alone be the glory in all things.
This is, once more, something all Christians would claim, either explicitly or implicitly. In all areas of life we are to give glory to God.
SO, in short PROCLAIM that you are REFORMED...