What are light, closely spaced members that span between beams or bearing walls called?

Light, closely spaced members that span between beams or bearing walls are referred to as joists. Joists are structural elements designed to support floors or ceilings, working in conjunction with beams or bearing walls to create a stable framework for a building. Their positioning is typically parallel to one another, and they help distribute loads to the beams or walls they rest upon.

In contrast, beams serve a different purpose; they are the larger horizontal members that carry the weight from joists and other loads. Lumber is a general term for processed wood used in construction, which can include various types of materials; thus, it doesn't specifically refer to the function described in the question. Columns are vertical structural elements that support beams and the loads above them, rather than spanning between two supports as joists do. This differentiation in roles and functions solidifies why joists are the correct term for closely spaced members between beams or bearing walls.