What is Basel I?

Legal Definition
Basel I is the round of deliberations by central bankers from around the world, and in 1988, the Basel Committee on Banking Supervision (BCBS) in Basel, Switzerland, published a set of minimum capital requirements for banks. This is also known as the 1988 Basel Accord, and was enforced by law in the Group of Ten (G-10) countries in 1992. A new set of rules known as Basel II was later developed with the intent to supersede the Basel I accords. However they were criticized by some for allowing banks to take on additional types of risk, which was considered part of the cause of the US subprime financial crisis that started in 2008. In fact, bank regulators in the United States took the position of requiring a bank to follow the set of rules (Basel I or Basel II) giving the more conservative approach for the bank. Because of this it was anticipated that only the few very largest US Banks would operate under the Basel II rules, the others being regulated under the Basel I framework. Basel III was developed in response to the financial crisis; it does not supersede either Basel I or II, but focuses on different issues primarily related to the risk of a bank run.
-- Wikipedia