meg(a)- Also megalo-.
Large or great; a factor of one million; a factor of 220.
[Greek megas, megal-, great.]
The sense of largeness appears in megaphone (Greek phōnē, sound, voice), a device that amplifies and directs the voice; megalithic (Greek lithos, stone), of prehistoric monuments made of or containing megaliths, large stones; megatherium (Greek thērion, animal), an extinct giant ground sloth; megastore, a very large retail store.
This sense is also the usual one for megalo-. Examples include megalomania (literally a passion for big things), an obsession with the exercise of power, especially in the domination of others; megalopolis (Greek polis, city), a very large, heavily populated city or urban complex; megaloblast, a large, abnormally developed red blood cell typical of certain forms of anaemia.
Mega- is also used as one of the standard SI (Système International) multiples (see the entry Words for multiples), indicating a factor of one million, as in the units megahertz, megawatt, megohm, and megaton; other examples include megabuck, a million dollars (or more loosely any very large dollar sum), and megadeath, a unit used to count the casualties of nuclear war, equal to the deaths of one million people. Since the 1980s, mega has become an adjective in its own right for something very large or excellent, or as a general-purpose intensifier.
In computing, mega- usually represents the binary multiple 220 or 1,048,576, as in megabit and megabyte, though these and related terms are also used more loosely in the sense of one million.