How can something with only one cell be considered to be an organism?

Hint: Since they can use energy (ATP), expand, replicate, and perform basic functions required to live, single-celled organisms are called organisms.

Complete answer:
This is a very unprofessional answer, but it takes a rational point of view. To live and carry out essential life functions, every organism requires energy. To conduct these functions, mitochondria in these cells use ATP (which are energy molecules). This can be achieved by even one neuron.
Cells will both split. This is commonly recognized as replication, and binary fission, mitosis, meiosis, etc will come under them. They are all ways of cell division that help categorize living organisms as single-celled organisms.
This is only a brief description of why species are called organisms with one cell.

While certain prokaryotes exist in colonies, they are not cells with various roles that are specialized. These species live together and all life cycles must be carried out by each cell to survive. Also the simplest multicellular species, by comparison, have cells that depend on each other to live.
There is a unicellular life-cycle period in most multicellular organisms. For example, gametes are reproductive single cells of multicellular species. Furthermore, in the evolution of life, multicellularity seems to have formed independently several times..

Note: Some cells, including Dictyostelium discoideum, are partly unicellular. In comparison, multinucleate cells, such as Caulerpa, Plasmodium, and Myxogastria may be unicellular.
The precursors of today's unicellular species were primitive protocells. While the origin of life is still largely a mystery, early RNA molecules may have been the basis for catalyzing organic chemical reactions and self-replication in the presently dominant theory, referred to as the RNA universe hypothesis.