An Instruction Set Architecture (ISA) is a set of instructions that a computer's central processing unit CPU can execute. It defines the operations, data types, registers, and addressing modes that the CPU can use to perform tasks. ISAs serve as the interface between software and hardware, allowing programmers to write code that the CPU can understand and execute. Different types of ISAs exist, such as RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer). RISC focuses on a small set of simple instructions for efficiency, while CISC includes a larger set of more complex instructions to perform tasks in fewer lines of code.