What term best describes a joint that allows for slight movement between masonry units?

The term that best describes a joint that allows for slight movement between masonry units is known as an expansion joint. This type of joint is specifically designed to accommodate the thermal expansion and contraction of materials due to temperature changes. When masonry warms, it expands, and an expansion joint provides enough space to prevent cracking or structural issues as the materials shift.

Expansion joints are critical in maintaining the integrity of masonry structures, particularly in environments where temperature fluctuations are common. By allowing movement, they help avoid stress and potential damage to the surrounding masonry units. This is essential in long walls or large structures where the scope for movement can be significant.

The other types of joints serve different purposes. Control joints, for instance, are used to manage cracking by providing a designated location for cracks to occur, while head joints refer to the vertical joints between the ends of masonry units. Work joints are temporary joints that occur during the construction process when work is paused and allow for continuity in the bonding material. Each plays a specific role, but when it comes to allowing for slight movement specifically due to thermal conditions, the expansion joint is the most fitting choice.