A semiautonomous, self replicating organelle that occurs in varying numbers, shapes, and sizes in the cytoplasm of virtually all eukaryotic cells. It is notably the site of tissue respiration.
Comment
Some anaerobic or microaerophilic organisms (e.g. Entamoeba histolytica, Giardia intestinalis and several Microsporidia species) do not have mitochondria, and contain mitochondrion-related organelles (MROs) instead, called mitosomes or hydrogenosomes, very likely derived from mitochondria. To annotate gene products located in these mitochondrial relics in species such as Entamoeba histolytica, Giardia intestinalis or others, please use GO:0032047 'mitosome' or GO:0042566 'hydrogenosome'. (See PMID:24316280 for a list of species currently known to contain mitochondrion-related organelles.)