The Secret Avengers was founded by Captain America in response to the Superhuman Registration Act, set up precisely for those who wanted to fight against Iron Man's initiative for superhuman registration. After the death of Captain America at the hands of a sniper, the group known as the Secret Avengers has been disbanded and the majority of the core members have joined the underground Avengers. But, recently Steve Rogers reformed the team.
"Secret Avengers" is used here for reference only. The term is not used by the inhabitants of the Marvel Universe.