An akshauhini (Sanskrit: अक्षौहिणी akṣauhiṇī) is described in the Mahabharata (Adi Parva 2.15-23) as a battle formation consisting of 21,870 chariots (Sanskrit ratha); 21,870 elephants (Sanskrit gaja); 65,610 horses (Sanskrit turaga) and 109,350 infantry (Sanskrit pada sainyam). Thus one akshauhini consisted of 218,700 warriors (not including the charioteers, who didn't fight). The ratio is 1 chariot : 1 elephant : 3 cavalry : 5 infantry soldiers. In each of these large number groups (65,610, etc.), the digits add up to 18.
It is mentioned in the Mahabharata that in the Kurukshetra War the Pandava army consisted of seven akshauhinis (1,530,900 warriors), and the Kaurava army had eleven akshauhinis (2,405,700 warriors).
Kuru Army is a coalition of 11 akshauhinis formed by the kingdom of Hastinapura in alliance with races like the Samsaptakas, Trigartas, the Narayana army, the Sindhu army and Shalya of Madra.
Pandava Army is a coalition of 7 akshauhinis, primarily the Panchala and Matsya forces, the Rakshasa forces of Bhima's son, Ghatotkacha and Vrishni -Yadava heroes.
The 4 types of units that make up an akshauhini can also be seen in Chaturanga, the predecessor of chess.