Source : Free On-Line Dictionary of Computing

progressive coding
     
         (Or "interlacing") An
        aspect of a {graphics} storage format or transmission
        {algorithm} that treats {bitmap} {image} data non-sequentially
        in such a way that later data adds progressively greater
        {resolution} to an already full-size image.  This contrasts
        with {sequential coding}.
     
        Progressive coding is useful when an image is being sent
        across a slow communications channel, such as the {Internet},
        as the low-resolution image may be sufficient to allow the
        user to decide not to wait for the rest of the file to be
        received.
     
        In an interlaced {GIF89} image, the {pixels} in a row are
        stored sequentially but the rows are stored in interlaced
        order, e.g. 0, 8, 4, 12, 2, 6, 8, 10, 14, 1, 3, 5, 7, 9, 11,
        13, 15.  Each vertical scan adds rows in the middle of the
        gaps left by the previous one.
     
        {PNG} interlaces both horizontally and vertically using the
        "{Adam7}" method, a seven pass process named after Adam
        M. Costello.
     
        Interlacing is also supported by other formats.  {JPEG}
        supports a functionally similar concept known as {Progressive
        JPEG}.  [How does the algorithm differ?]
     
        {JBIG} uses {progressive coding}.
     
        See also {progressive/sequential coding}.
     
        ["Progressive Bi-level Image Compression, Revision 4.1",
        ISO/IEC JTC1/SC2/WG9, CD 11544, 1991-09-16].
     
        (2000-09-12)