Mitochondria are called semiautonomous organelles.
Mitochondrion shows a degree of autonomy or independence in their functioning.
However, mitochondria are not fully autonomous because their structure and functioning are partially controlled by nucleus of the cell and availability of materials from cytoplasm. So, it is more appropriate to called them semiautonomous organelles.
Semi autonomy of mitochondria shows the following characteristics:
Mitochondria have their own DNA which can replicate independently.
All three types of RNAs are present in mitochondrial matrix, e.g., mRNA, tRNA and rRNA.
Mitochondrial matrix possesses its own ribosome.
Mitochondria synthesize of their own structural proteins.
New mitochondria develop by division of pre-existing mitochondria.