There is no hard and fast rule for buying binoculars, but a compact binocular is a small binocular, small enough to fit in a large pocket or a purse. What makes the binocular small is the use of smaller front lenses the second binocular number is the size of the front lenses in millimeters). In general, any binocular with a second binocular number of 26 or less is a compact binocular. There are a few compact binoculars with a second number of 28, but these 28s, though wonderful to handle and use, really push the limit in terms of size. You may have problems getting them in your pocket or your purse.
I've had a fascination with compact binoculars most of my life. To be sure, I own and use everything in terms of binocular size from the smallest to the largest giant binoculars, but compacts have always appealed to me. Just something about being able to take a binocular with you, wherever you go and not have it get in the way. Compact binoculars fit my on the go lifestyle, perfectly. Whether I am bicycling, cross country skiing, canoeing, hiking or just shopping or out for an evening walk, the small size of a compact binocular makes it my constant companion. As a binocular expert and user for over forty years, I can tell you that compact binoculars have their advantages and disadvantages, just like any other binocular. They may or may not be the perfect binocular for you.
Just as in full size binoculars, compact binoculars come in two basic prism types - compact porro binoculars and roof prism - and these prism types affect the body shape and, therefore, the handling qualities of the binocular. Again, there are pluses and minuses to each design.