Definition
'One-celled' is simply another term for 'single-celled.' It describes organisms that consist of a single cell performing all necessary life functions. Think of bacteria, some algae, and protozoa. It's just a simpler, more direct way of saying the same thing. While 'single-celled' is more common in scientific writing, 'one-celled' is perfectly acceptable. It's like saying 'one' instead of 'single', less formal. Each one-celled organism is a complete and independent living unit.