Part 3 of the definition

Wiktionary Word definitions in Wiktionary
n. The mental act by which something is forgotten. vb. (present participle of forget English)

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Forget \For*get"\, v. t. [imp. Forgot ( Forgat , Obs.); p. p. Forgotten , Forgot ; p. pr. & vb. n. Forgetting .] [OE. forgeten, foryeten, AS. forgietan, forgitan; pref. for- + gietan, gitan (only in comp.), to get; cf. D. vergeten, G. vergessen, Sw. f["o]rg["a]ta, ...

WordNet Word definitions in WordNet
See forget

Wikipedia Word definitions in Wikipedia
Forgetting or disremembering is the apparent loss or modification of information already encoded and stored in an individual's long term memory . It is a spontaneous or gradual process in which old memories are unable to be recalled from memory storage. ...