Equity is the law that developed in England in the Court of Chancery prior to 1873 (at which point the Judicature Act gave the courts jurisdiction in both common law and equity). It developed largely as a reaction to the strict rules enforced in the common law courts. Although initially more flexible than the common law (thus prompting the famous remark by John Selden that “Equity varies with the length of the Chancellor’s foot”) equity has become more fixed over time.
In the United States the Federal government and most of the State governments have merged their equitable and common law courts. But principles of equity remain very important in many legal doctrines, such distinguishing when a plaintiff has a right to a jury under the U.S. Constitution.