المعنى الاصطلاحي :

Asking Allah to pardon one's sins and to protect him from their consequences.

الشرح المختصر :

"Istighfaar" (Seeking forgiveness) is a great act of worship, whether it is done because of a sin committed or not. "Istighfaar" may be mentioned (in scriptures) individually, in which case it has the same meaning as "tawbah" (repentance); i.e. quitting the sin spiritually and physically. It may also be mentioned together in combination with "tawbah", in which case it means asking Allah for protection against the evil consequences of past sins. "Tawbah" means quitting the sin and asking Allah for protection against what one fears to commit in the future of evil deeds. There are several reported formulas for "istighfaar", such as: "Rabbi-ighfir lee" (O my Lord, forgive me); "astaghfirullah" (I ask Allah to forgive me); as well as other formulas.

التعريف اللغوي المختصر :

"Istighfaar": asking for "maghfirah" (forgiveness). The original meaning of "maghfirah" is covering and concealing, and it is used to mean overlooking the sin and not holding the sinner accountable for it.