A base address is a unique location in primary storage (or main memory) that serves as a reference point for other memory locations called absolute addresses.
In order to obtain an absolute address, a specific displacement (or offset) value is added to the base address. In primary storage, all addresses literally comprise fixed-length sequences of bits that stand for positive whole numbers usually expressed in hexadecimal form. For example, a base address might indicate the beginning of a program loaded into primary storage. The absolute address of each individual program instruction could be specified by adding a displacement to the base address.