Unit cell is the smallest portion of a crystal lattice.
A unit cell is characterized by:
(i) its dimensions along the three edges a, b and c. These edges may or may not be mutually perpendicular.
(ii) angles between the edges, α (between b and c), β (between a and c),γ (between a, and b). Thus, a unit cell is characterised by six parameters, a, b, c, α, β and γ.