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In the Pāli Canon, a collection of the Buddha's earliest teachings, the term Appamāda is quite significant and the essence of the meaning can not be captured with one English word. "Heedfulness", "Diligence", "Conscientiousness", capture some aspects of Appamāda. To really get an understanding of the range of Appamāda, we have to examine the word in the context of key sutta (Buddha's discourse) passages. The important point is that Appamāda ("Heedfulness") not only leads to perfection of ethical conduct (which on its own only leads to heavenly rebirth), but to all the various skillful methods taught in the Buddha's dispensation that culminate in the realization of nibbāna (transcending the entire cycle of endless rebirth and death). Appamāda ("Heedfulness") is the source of all skillful qualities. AN 10.15 has a series of similes all with this refrain: ...all skillful qualities are rooted in heedfulness, converge in heedfulness, and heedfulness is reckoned the foremost among them. In the passage below, the Buddha, exhorting his followers for the last time before dying, advised them to be "Heedful", and then demonstrated complete mastery of the 9 meditative attainments in forward and reverse order. In other sutta passages such as the chapter of Appamāda of the Dhammapada, the context makes it clear that Appamāda ("Heedfulness") is to be developed in a way leading to mental mastery, meditative attainments, culminating in nibbāna.
Before the Buddha passed (death) into final nibbāna, his last advice to the order of monks: [SN 6.15 pari-nibbāna Sutta] Atha kho bhagavā bhikkhū āmantesi: handa dāni bhikkhave āmantayāmi vo vayadhammā saṅkhārā appamādena sampādethāti. Ayaṃ tathāgatassa pacchimā vācā.
Then the Blessed One addressed the monks, "Now, then, monks, I exhort you: All fabrications are subject to decay. Bring about completion by being heedful (Appamāda)." Those were the Tathāgata's last words. (The Buddha then goes into 9 meditative attainments in forward and reverse order before passing)