Secrecy

Mid-frequency: not on every page, but regular in books and magazines.

More common than 似 (Resemble), less common than 寝 (Lie Down).

Under a roof on top, a heart hides behind a mountain below -- a heart hidden behind a mountain under a closed roof maintains perfect secrecy.

In words

密かひそか secret;private;surreptitiousN1

密度みつど densityN1

密集みっしゅう crowd;close formation;denseN1

密 is rank 694 of 2,608 in published Japanese.