Gynecomastia is the enlargement of male breast tissue. It can also be developed depending on increase in fat tissue there, as can be developed depending on the enlargement of breast tissue.
As can be developed temporarily in the puberty period, it may also result from some drugs used (Digoxin, estrogen, marijuana). Sometimes, it can grow without being associated to an underlying cause. In the end, it may occur a condition that disturbs a person's aesthetical appearance.