1.  megabyte(n = noun.quantity) m, mb - a unit of information equal to 1000 kilobytes or 10^6 (1,000,000) bytes;
is a kind of computer memory unit
is a part of g, gb, gigabyte
has parts: k, kb, kilobyte

2.  megabyte(n = noun.quantity) m, mb, mebibyte, mib - a unit of information equal to 1024 kibibytes or 2^20 (1,048,576) bytes;
is a kind of computer memory unit
is a part of g, gb, gib, gibibyte, gigabyte
has parts: k, kb, kib, kibibyte, kilobyte

megabyte, n. Computing 1,048,576 (i.e. 2(20).) bytes as a measure of data capacity, or loosely 1,000,000.

Usage:
Abbr.: MB.