The term somatic means 'of the body',, relating to the body. In medicine, somatic illness is bodily, not mental, illness. The term is often used in biology to refer to the cells of the body in contrast to the germ line cells which usually give rise to the gametes (ovum or sperm). These somatic cells are diploid containing two copies of each chromosome, whereas the germ cells are haploid as they only contain one copy of each chromosome.
Although under normal circumstances all somatic cells contain identical DNA, they develop a variety of tissue-specific characteristics. This process is called differentiation, through epigenetic and regulatory alterations. The grouping of like cells and tissues creates the foundation for organs.

Somatic mutation

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

s are changes to the genetics of a multicellular organism which are not passed on to its offspring through the germline. Many cancer

Cancer

Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

s are somatic mutations.

Somatic is also defined as relating to the wall of the body cavity, particularly as distinguished from the head, limbs or viscera.

It is also used in the term somatic nervous system

Somatic nervous system

The somatic nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles...

 which is the portion of the vertebrate

Vertebrate

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 nervous system

Nervous system

The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

which regulates voluntary movements of the body.