CISC stands for Complex Instruction Set Computer. It refers to a type of computer architecture that is characterized by a large set of complex instructions. In CISC architecture, instructions can perform multiple operations and can have variable execution times.

CISC Architecture:
CISC architecture was developed in contrast to RISC (Reduced Instruction Set Computer) architecture. The main idea behind CISC was to design a processor that could execute complex instructions in a single cycle, reducing the number of instructions required to perform a task. This approach aimed to simplify programming and improve code density.

Key Features of CISC Architecture:
1. Large Instruction Set: CISC processors have a large number of instructions that can perform complex operations. These instructions are generally multi-byte and can have variable lengths.

2. Variable Execution Times: Instructions in CISC architecture can take different amounts of time to execute. This is because some instructions may involve more complex operations that require more clock cycles to complete.

3. Memory-to-Memory Operations: CISC processors often support instructions that can directly operate on memory locations, allowing for more flexible data manipulation.

Advantages of CISC Architecture:
1. Code Efficiency: CISC instructions can perform complex operations in a single instruction, reducing the number of instructions required to accomplish a task. This improves code density and can lead to more efficient program execution.

2. Flexibility: CISC architecture allows for a wide range of instructions, providing flexibility to programmers and enabling them to write more complex programs.

3. Legacy Support: CISC processors are often designed to be backward compatible, allowing them to run older software without any modifications.

Disadvantages of CISC Architecture:
1. Complexity: The large number of instructions and variable execution times make the design and implementation of CISC processors more complex compared to RISC processors.

2. Slower Clock Speeds: CISC processors often have slower clock speeds due to the complexity of the instructions and the need for more circuitry to handle them.

In conclusion, CISC stands for Complex Instruction Set Computer, which is a type of computer architecture characterized by a large set of complex instructions. It aims to simplify programming and improve code efficiency by allowing complex operations to be performed in a single instruction. However, the complexity of CISC architecture can lead to slower clock speeds and more complex designs.