Joists are any of the small timbers or metal beams. These small beams ranges parallel from wall to wall in a structure to support a floor or ceiling.
Joists are less important beams that are closely spaced and are frequently having truss-type webs.
These are closely spaced smaller beams.. In most of cases joists rest on secondary beams. With the presence of joists, the strength requirements of the roof sheathing or slab are greatly reduced.
Steel structures by Zahid Ahmad siddiqi and Muhammad ashraf
Meriam webster dictionary