The voluntary prevention of conception through arrested coition or the use of contraconceptives, for the purpose of limiting the number of offspring.
As commonly used the term means the absolute prevention of pregnancy.
While the Catholic Church does not urge married persons to beget the largest possible number of children, and does not sanction the intemperate use of marriage, she does condemn each deliberate act of birth control as intrinsically evil (S.Off., 21 May 1851; 19 April 1853; S.Pren., 13 November 1901).
In cases of ill health or poverty she insists on marital abstinence.
The Catholic doctrine that birth control is essentially wrong, is not a mere disciplinary measure, like the law of clerical celibacy, which can be abrogated or modified by the Church.
It is a definition of the law of God, which no power, not even the Church itself, can abrogate or contravene.
In the Divine plan the primary purpose of the marital act is the procreation of offspring, and its secondary purposcs are the cementing of conjugal love and the allaying of concupiscence.
As birth control defeats the primary purpose of the marriage relation, it is opposed to the Divine Will, which the Church must sustain.
This teaching is reenforced by other weighty considerations, e.g., birth control undermines the respect of husband for wife, and vice versa, and thereby increases unhappiness among married people and the consequent number of divorces.
The essential evil of birth control, however, consists in frustrating the primary purpose of marriage, the propagation of the species.
New Catholic Dictionary