What is the tightness or looseness of thread called?

The tightness or looseness of a thread is referred to as "tension." In sewing or textile contexts, proper tension is crucial for achieving even stitches and ensuring the fabric is not too tight or too slack. In other applications, such as in mechanical threading or fastening, tension can affect the strength and durability of the connection. Adjusting the tension can help achieve the desired performance in various tasks.