The word 'forgetfulness' is a noun, a word for a lapse of memory; a word for a thing.

A noun functions as the subject of a sentence or a clause, and as the object of a verb or a preposition.

Examples:

• My forgetfulness caused me to miss the appointment. (subject of the sentence)
• They are fond memories which forgetfulnessimproves. (subject of the relative clause)
• I blame his forgetfulness as the cause of the problem. (direct object of the verb 'blame')
• The professor is well known for forgetfulness. (object of the preposition 'for')