Screws and bolts have clear professional terms, have a clear distinction, and should not be called arbitrarily.
There are more than 100 national standards for the classification of screws and bolts. There are 92 standards for screws alone. There are too many standards and it is not convenient to remember. They are not listed one by one, but there are rules to follow. You can simply distinguish as follows:
All fasteners composed of round head and threaded rod are called screws. It must be tightened with a screwdriver and hexagonal tools, such as slotted cylinder head screws, slotted countersunk head screws, cross recessed countersunk head screws, hexagon socket head screws, hexagon socket countersunk head screws, etc.; various types of tightening without round heads Set screws; various positioning screws, non-stripping screws, axis screws; various knurled screws (with knurling without tools to tighten); there are wood screws and self-tapping screws, etc.
Fasteners with hexagonal head, square head, and threaded rod are called bolts. There are no hexagonal heads, but the two ends with a smooth rod in the middle of the thread are called studs (double-ended studs), and the whole body is a threaded screw. In addition, there are countersunk square neck bolts, T-slot bolts, anchor bolts, U-shaped bolts. The bolt tightening tools are torque wrenches, fixed wrenches of various shapes with a constant opening size, and adjustable wrenches with a variable opening size. Hydraulic tightening bolts require accessories such as hydraulic pumps.
In short: the fasteners tightened with a screwdriver and hexagonal tools are screws, and the fasteners tightened with a wrench are bolts.
Note: It cannot be distinguished by the nominal diameter and length. For example, an M48 cylindrical head fastener with a length of 300mm is defined as a hexagon socket head screw, not a bolt, according to the GB/70.1-2008 standard.