A 64-bit signed integer is a data type used in computer programming to represent whole numbers. It can store values ranging from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807. The "64-bit" part indicates that it uses 64 bits of memory, allowing for a large range of values compared to smaller integer types. The term "signed" means that the integer can represent both positive and negative numbers. This is achieved by using one bit to indicate the sign (positive or negative) and the remaining bits to represent the magnitude of the number. This format is commonly used in programming languages like C, Java, and Python.