A desert is a dry, barren landscape that receives very little rainfall. The term 'desert, mostly' refers to an area that is mostly desert, but may have some vegetation or other features. The crossword solution for the term 'desert, mostly' is 'SAND, ' which is a common feature of deserts. SAND is composed of small particles of rock and mineral matter that are eroded by wind and water. It is often found in large dunes or drifts in desert regions.