So, herein I propose a possible format for such a dictionary, starting with a word I have been working on myself recently. I welcome comments and feedback on the usefulness of such a project.
From: Middle English sacrement, sacrament, from Anglo-French and Late Latin; Anglo-French, from Late Latin sacramentum, from Latin, oath of allegiance, obligation, from sacrare to consecrate
Date: 13th century
1 a: a Christian rite (as baptism or the Eucharist) that is believed to have been ordained by Christ and that is held to be a means of divine grace or to be a sign or symbol of a spiritual reality b: a religious rite or observance comparable to a Christian sacrament
2 capitalized a: communion b: blessed sacrament
3 something likened to a religious sacrament
Hexham's Concise Dictionary of Religion
A Rite in which God (or Gods) is (are) uniquely active. Augustine of Hippo defined a Christian sacrament as "a visible sign of an invisible reality." The Anglican Book of Common Prayer speaks of them as "an outward and visible sign of an inward and invisible grace." Examples of sacraments would be baptism and the mass.
For Roman Catholics, recognized sacraments include baptism, confirmation, ordination of clergy, the Eucharist (communion), confession, matrimony, and unction for the dying. Protestant churches largely retain only baptism and communion among their sacraments. Sikhs and Hindus recognize as many as 42 samskāras. These ritualistic rites of passage celebrate accomplishments of life and prepare the individual's mind and body for full membership in the community. While these represent the recognized form of sacraments, what defines something as a sacrament more generally, that is, for someone who does not adhere to a specific religious tradition?
A common thread running through the many definitions one finds of the term is that a sacrament is an act that bestows grace through a material vehicle on a recipient, where grace (in the Christian theology) is God's free expression of love. So, by consuming the wafer and the wine, God transfuses the faithful with his spiritual energy. The touch of water during baptism blesses a child with the enabling power of God. Generally, a sacrament requires the right matter (such as the wafer and the wine), the right form of action of ritual or ceremony, and the right intention on the part of the participant. The sense of such requirements prevents mundane or simply habitual practices from acquiring the important status of sacrament. Sacraments show that the grace of God lies not always in the invisible and the unknowable, but can work through specific matter, people, and institutions.
An additional attribute of sacraments recognized by various faith traditions is that, generally, salvation or the achievement of religious consciousness requires the performance of sacraments in a person's life. Now, if one does not adhere to the belief in a god who would create any soul destined to an eternity of damnation, then no specific act is required to attain salvation or ultimate consciousness, which is inevitably inherent in every person.
Atheist Definition: A sacrament is a ritual or ceremony (perhaps related to significant life stages or rites of passage) during which one seeks, receives, and accepts through a physical act and form the sensation of unexpected energy from unknown sources, or the revelation of deep personal or universal understanding.