You know that round thing on your car engine that has a v-belt running on it that drives an alternator or water pump or airconditioning compressor, it's also a sheave. but a lot of people would call it a pulley. Thats ok most people will understand what you mean. When you put a sheave between a couple of pieces of material so that it becomes portable it turns into a block.The round part with the groove that a line or belt runs on is a sheave, you are right. But it is also correct to call a block a block.