Original: Most often today, NAT is used in conjunction with network masquerading (or IP masquerading) which is a technique that hides an entire address space, usually consisting of private network addresses (RFC 1918), behind a single IP address in another, often public address space.
Simplified: Masquerading is the mechanism that hides an entire address space, usually consisting of private network addresses (RFC 1918), behind a single IP address usually in the public domain address space.