Istighfar is a servant's seeking of forgiveness from Allah for the sins committed. The shortest and most common form, "Astaghfirullah", means "I seek forgiveness from Allah."
Sayyid al-Istighfar (The Master of Forgiveness)
Our Prophet (PBUH) taught the most virtuous prayer for forgiveness known as Sayyid al-Istighfar. It is a comprehensive prayer acknowledging Allah's lordship and our servanthood.