A metafile is a file containing information that describes or specifies another file.
Microsoft pushes out critical patches
Microsoft has warned of critical security flaws...(ComputerWeekly.com)
M3U (Moving Picture Experts Group Audio Layer 3 URL)
M3U (Moving Picture Experts Group Audio Layer 3...(WhatIs.com)
Microsoft uses this term for its Windows Metafile (WMF) format. A WMF file contains a sequence of graphical-device-interface (GDI) function calls ("commands" to the Windows operating system ) that results in the presentation of a graphic image. Some of the function calls are equivalent to vector graphics statements and others identify stored bitmap or literal specifications of which bits to illuminate ( raster graphics images). Using WMF files rather than already-built bitmaps saves space when many bitmaps are used repeatedly by different components of the operating system or of an application. The WMF file assumes a 16-bit operating system. Microsoft has an enhanced metafile (EMF) format for its 32-bit operating systems. Microsoft's clipboard (CLP) file can contain a WMF file, an EMF file, or a bitmap (BMP) format file.
Another example of a metafile is the Computer Graphics Metafile (CGM). The CGM file format is a standard ( American National Standards Institute -approved) format that can be used on any operating system that supports it (unlike the WMF format which is designed only for Windows). The CGM file is commonly used in CAD and presentation graphics applications.
In general, there are tools (such as HiJaak) that will convert the WMF, EMF, and CGM formats into other formats.