Anicca-saññā: Perception of impermanence, is defined in the Girimananda Sutta A.X. 60 as meditation on the impermanence of the five clusters of clinging:
Though, with a faithful heart, one takes refuge in the Buddha, his Dhamma-Teaching and the Sangha Community of Bhikkhus; or with a faithful heart observes the rules of morality, or develops a mind full of loving-kindness, far more advantageous is it if one cultivates the perception of impermanence, be it only for a moment A.X. 20. See A. VI, 102; A. VII, 48; Ud. IV, 1; S. XXII, 102.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.