The major structural component of spiral, lenticular, and some irregular galaxies, containing stars and (for spirals and irregulars) gas and dust, orbiting the galaxy’s centre. The thickness of the disk is small in relation to its diameter. In our own Galaxy the disk extends about 80 000 l.y. from the galactic centre, while its total thickness is about 1500 l.y. as measured by the distribution of older stars, and only 600 l.y. for young stars, gas, and dust.