na-adjective, noun which may take the genitive case particle 'no'
forgetful; prone to forgetting; negligent
Describes a tendency to forget things or be inattentive. Often used with ～になる or as a な-adjective modifying a person or behavior.
最近、忘れがちになってきた。
Lately I've become forgetful.
彼は忘れがちな性格だ。
He has a forgetful nature.
忘れっぽい is a more colloquial i-adjective meaning 'forgetful' or 'absent-minded', while 忘れがち is slightly more formal and emphasizes a tendency or habit.
Compound of 忘れ (continuative form of 忘れる 'to forget') + がち (suffix meaning 'apt to do' or 'tend to').