The word "sloth" is a translation of the Latin term acedia (Middle English, accidie) and means "without care" . Sloth is one of the seven deadly sins. It is the most difficult sin to define, and to credit as sin. In the Philokalia , the word dejection is used instead of sloth. The demon Belphegor is often associated with sloth. It is also one of the five hindrances in Buddhism. Sloth refers to a peculiar jumble of notions, dating from antiquity and including mental, spiritual, pathological, and physical states. It may be defined as absence of interest or habitual disinclination to exertion.

Saint Thomas Aquinas defined sloth as "sorrow about spiritual good". Spiritually , acedia first referred to an affliction attending religious persons, especially monks, wherein they became indifferent to their duties and obligations to God. Mentally, acedia, has a number of distinctive components of which the most important is affectlessness, a lack of any feeling about self or other, a mind-state that gives rise to boredom, rancor, apathy, and a passive inert or sluggish mentation, Physically, acedia is fundamentally associated with a cessation of motion and an indifference to work; it finds expression in laziness, idleness, and indolence. Sloth includes ceasing to utilize the seven gifts of grace given by the Holy Spirit (Wisdom, Understanding, Counsel, Knowledge, Piety, Fortitude, and Fear of the Lord). Unlike the other capital sins, which are sins of committing immorality, sloth is a sin of omitting responsibilities. Emotionally and cognitively, the evil of acedia finds expression in a lack of any feeling for the world.

Chaucer counted the characteristics of the sin to include despair, somnolence, idleness, tardiness, negligence, indolence, and wrawnesse, the last variously translated as "anger" or better as "peevishness". Acedia in Chaucer's view is the enemy of every source and motive for work. Dante describes acedia as the failure to love God with all one's heart, all one's mind and all one's soul; to him it was the middle sin, the only one characterised by an absence or insufficiency of love. Some scholars have said that the ultimate form of acedia was despair which leads to suicide.

Although sloth is more akin to apathy and inactivity, many English speakers narrow it to mean simple laziness, about which the Bible makes many comments. (Never be lazy, but work hard and serve the Lord enthusiastically. Romans 12:11; Lazy people consider themselves smarter than seven wise counselors. Proverbs 26:16; Lazy people irritate their employers, like vinegar to the teeth or smoke in the eyes. Proverbs 10:26).