Answer:
Prokaryotes
Step-by-step explanation:
Organism without a definite nucleus are called prokaryotes.
Prokaryotes are the defined as the organism that lack nucleus and are asexual, reproducing without fusion of gametes. Generally, prokaryotes are single-celled organisms. Prokaryotes are categorized in two categories - the bacteria and the archaea. Prokaryotes also lack some other organelles such as mitochondria or other membrane-bound organelles.
Due to lack of nucleus, DNA of prokaryotes is present in cytoplasm of the organism.
Hence, the organism without a definite nucleus is prokaryote.