Mono no aware (物の哀れ), literally "the pathos of things", and also translated as "an empathy toward things", or "a sensitivity to ephemera", is a Japanese term for the awareness of impermanence (無常 mujō), or transience of things, and both a transient gentle sadness (or wistfulness) at their passing as well as a longer, deeper gentle sadness about this state being the reality of life.

art source: http://www.pixiv.net/member_illust.php?mode=medium&illust_id=43918947

niconico sources for individual songs (in order):
>sm13920294
>n/a (album exclusive)
>sm23265517
>sm9893923
>sm6529016
>sm25672022
>sm11209477
>sm1204327
>nm17567478

9 tracks