Unicellular means one cell or single cell.
Unicellular organisms only have one cell - and include Prokaryotes, most protists and some fungi.
Unicellular organisms can died, but often they reproduce by fissioning into two new cells. Since neither cell is considered the original, the old cell never really dies.
They are multi
The difference is that unicellular organisms are made up of one cell, while multicellular are made up of many cells.