Falsification is the act of deliberately lying about or misrepresenting something. If you write a note to your teacher excusing your absence the day before and claim it was written by your dad, that's falsification.
People commit falsification whenever they present something that's forged or altered so that its information is false. Changing your grades on your report card before your parents sign it is falsification — and likely to get you in big trouble! This noun comes from the verb falsify, "alter so as to mislead," from the Latin root falsus, "erroneous, mistaken, or false."