What is the definition of sacrament?

In the English dictionary the word Sacrament is described as -

'A visible sign of an inward grace, especially one of the solemn Christian rites considered to have been instituted by Jesus Christ to symbolize or confer grace: The sacraments of the Protestant churches are baptism and the Lord's Supper; the sacraments of the Roman Catholic and Greek Orthodox churches are baptism, confirmation, the Eucharist, matrimony, penance (confession), holy orders, and extreme unction (anointing of the sick).

It is also linked with the words Holy Sacrament (the Eucharist or Lord's Supper) and can be interpreted as something that is regarded as possessing a sacred character or mysterious significance.'

It is basically a religious term which refers to a ritual done in church, that is commemorative of the actions of their god (beliefs).  This preserves and renews the faith of the gathering and their belonging within that church.

Different religions have different sacraments.  The following are the seven sacraments of the Roman Catholic Church -

1. Baptism (Christening)

   

2. Confirmation (Chrismation)
   
3. Holy Eucharist
4. Penance (Confession)
5. Anointing of the Sick (Extreme Unction, also seen as part of the 'Last Rites')
6. Holy Orders
7. Matrimony (Marriage)

In Eastern and Oriental Orthodoxy they consider the above to be the 'major' sacraments however they do have additional ones -

1. Consecration of a Church

2. Monastic Tonsure

   

3. Burial of the Dead
   

And so there are slight changes for each religions but they are all important rituals for that specific religion.

Hai,

  A ceremony regarded as imparting spiritual grace, in particular

Gavara Matrimony