Sacramentals are objects used in the practice of the sacraments.
According to the Catechism of the Catholic Church, sacraments are
"efficacious signs of grace, instituted by Christ and entrusted to the Church, by which divine life is 'dispensed' to us. The visible rites by which the sacraments are celebrated signify and make present the graces proper to each sacrament. They bear fruit in those who receive them with the required dispositions."
Similarly in the Eastern Church,
Sacrament: The outward and visible part of religion, consisting of various ceremonies, words, and symbolisms, producing an invisible action by the Holy Spirit that confers grace on an individual. All Sacraments were instituted by Christ for the salvation of the believe.
In most Protestant Churches, two Sacraments are observed: Baptism and the Lord's Supper.
Sacramentals, therefore, are any objects that facilitate the practice of these rites. These objects may include specially blessed or adorned material objects, things, or actions that manifest and act as visible reminders of the respect for and particular importance of such rites.
Sacramentals may include Bibles, Chalices, icons, textiles, adornments, oils, prayers, chants, postures, signs made with the hands or body, locations, and other things especially set apart from those of a similar type for the purpose of practicing the sacrament that they facilitate.