It is the U.S. spelling of the word, “defence”. Defence is the action of defending something. Any kind of structure that is built in order to protect a building, country etc. against some sort of attack by the terrorists or some other erratic forces forms the defence. “Offence is the best form of defence”, is a famous proverb which suggests attack to be more preferable than simply safeguarding or defending ourselves. In this context, Sun Tzu had remarked, “Invincibility lies in the defence; the possibility of victory in the attack.” Defence is considered as the attempted justification of something. It also refers to the case presented by the person being accused or sued in a lawsuit.
In the context of sports, defence is the action of defending the goal or wicket, or the group of players who perform this role. Anything that is without defence is completely vulnerable and susceptible to attack. Each country has its own defence forces which are basically, the army, air force and navy. The defence forces protect their nation from any attacks.