In defining Calvinism, or any other theological tradition, we need to distinguish between an inclusive and exclusive definition. Put another way, to distinguish between Reformed essentials and Reformed distinctives. For example, belief in the final authority of Scripture is a Reformed essential, but not a Reformed distinctive inasmuch as this belief is shared in common with all other traditional Protestants.
In terms of what sets it apart from other theological traditions, Reformed theology is often defined by the so-called five points of Calvinism, popularized by the acronym of TULIP (=total depravity, unconditional election, limited atonement, irresistible grace, perseverance of the saints).
But although these are key ingredients, such a definition suffers from a couple of limitations: (i) it is rather reductionistic. One theologian (Leonard Coppes) offers a ten-point definition; (ii) it fails to distinguish between what is fundamental and what is instrumental.
In terms of its leading principle, we do better to paraphrase Wilhelm a Brakel: God, as the highest good, made a race of rational agents to share in his goodness. He foreordained the Fall of Adam and the cross of Christ so that his chosen ones would glory in his wisdom, mercy, and justice (Rom 9:17,21-22; 11:32; Gal 3:24; Eph 3:9-10; 2 Thes 1:10; Jn 9-12; Rev 11:13). From this there flows the five points of Calvinism, as well as other Reformed essentials and distinctives.