Here is the basic definition from Wikipedia:
Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order. Modern cosmology is dominated by the Big Bang theory, which brings together observational astronomy and particle physics.
Though the word cosmology is recent (first used in 1730 in Christian Wolff‘s Cosmologia Generalis), the study of the universe has a long history involving science, philosophy, esotericism, metaphysics, and religion. Related studies include cosmogony, which focuses on the origin of the Universe, and cosmography, which maps the features of the Universe. Cosmology is also connected to astronomy. However, they are contrasted in that while the former is concerned with the Universe in its entirety, the latter deals with individual celestial objects.