Separated compartments within a cell with specialized functions, e.g. nuclei (containing most of the genetic material), mitochondria (respiratory energy supply for the cell), chloroplasts (location of photosynthesis) etc.
Source:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 162 (https://doi.org/10.1351/pac199264010143)