Glossary
Half buy
In one line: A partial buy: cheaper guns and light armor, keeping part of your money.
What it means
In a half buy the team buys something decent (SMGs, strong pistols, light armor) but not everything, keeping enough money for a full buy next round. It sits between an eco and a force buy.
It works when having some guns gives you a real chance (for example against an enemy who did not full buy either) and the numbers still add up for the next round. If they do not add up, it is just a way to lose two rounds instead of one.
An example
After losing a round you have 3000 credits each. You buy Sheriffs or Spectres and light armor and keep about 1500: with the loss bonus, you reach a full buy next round.