The term for displaying a GIF in low resolution before the full image loads is interlacing. An interlaced GIF stores its raster rows in a special order so a rough version can appear before every line has arrived.
GIF interlacing uses four passes. The first pass presents widely spaced rows, followed by additional passes that fill in the gaps. Viewers therefore see a recognizable outline or coarse image that becomes more complete as decoding continues. This was especially useful when web pages were downloaded over slow modem connections.
Interlacing changes the order in which rows are stored and displayed; it does not reduce the image's final dimensions or create a separate low-resolution copy. The feature is optional, so GIF encoders can produce either interlaced or non-interlaced images.
“Progressive” is a related term often associated with JPEG and other formats, but GIF's standard terminology is interlacing. PNG also supports an incremental display method, using the Adam7 interlacing algorithm, though its pass structure differs from GIF's.