Spectre
SPECTRE is an abbreviation for Special Tactics and Recon, an organization of special agents for the Citadel Council in Mass Effect.
Organization
SPECTREs are individual agents authorized by the Council to take whatever actions they deem necessary to protect the interests of the Council races. SPECTREs typically operate independently of each other, gathering whatever resources (be they personnel or material) that they need to perform their duties. Individuals selected for SPECTRE status are typically distinguished members of the elite armed services of their homeworld governments.
A SPECTRE is only answerable to the Council, and when a SPECTRE is judged to have "gone rogue", the Council's will revoke their SPECTRE status. As anyone appointed to be a SPECTRE is an extraordinarily capable and dangerous individual, the Council will typically send another SPECTRE to apprehend the rogue agent.