Senaa means army. Army, now-a-days include army (who fight from land), navy (who fight from ships) and  -- (who fight from air planes).

In olden days, in India, a good king used to have Chaturanginee army. Chaturanginee means an army with four wings -
Chariot riders - who usd to fight riding on chariots
Elephant riders - who used to fight riding on elephants
Horse riders - who used to fight riding on horses
Foot soldiers - who used to fight on foot

There were many small kings also who did not have all these four wings of army with them. Normally people had these four wings in a certain proportion -

According to Mahaabhaarat, G-5-Prewar/21
One chariot was attached with 50 elephants,
each elephant was attached with 100 horses, and
each horse was attached to 7 foot soldiers.

500 chariots, as many elephants, 1,500 horses, 2,500 foot soldiers constituted a Senaa (army).
10 such Senaa constituted a Preetan, and
10 such Preetan constituted a Vaahinee.
But commonly Senaa, Vaahinee, Preetan, Dhwajinee, Chamu, Akshauhinee and Varuthinee all are used in the same sense.

250 men constitute a Pattee, and
3 Pattee made a Senaamukh or Gulm, and
3 Gulm made a Gan.
Duryodhan had 1,100 such Gun.