From The Dhamma Encyclopedia
Appamāda: Alertness, attentiveness, carefulness, non-laxity, earnestness, diligence, vigilance, is considered as the foundation of all advantageous progress.
Just as all the footprints of living beings are surpassed by the footprint of the elephant, and the footprint of the elephant is considered as the mightiest amongst them, just so have all the meritorious qualities alertness as their foundation, and alertness is considered as the mightiest of these qualities A. X, 15.
Cf. the Chapter on alertness Appamāda Vagga in Dhp., and the Buddha's last exhortation: Transient are all constructions. Be Alert & Train! appamādenasampādetha D. 16 - In the commentaries, it is often explained as the presence lit. 'non-absence' of awareness satiyāavippavāsa.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.