A pulley block with a single sheave (wheel). Used in rigging and mechanical contexts.
We changed the direction of the rope using a single-sheave pulley block.
滑車 is the general term for a pulley or block; 一つ目滑車 specifies a single-sheave block.
Compound of 一つ目 (hitotsume, 'one eye' or 'first') and 滑車 (kassha, 'pulley'). The 'one eye' refers to the single wheel or sheave in the block.