Cervical dilation is the opening of the cervix

Cervix

The cervix is the lower, narrow portion of the uterus where it joins with the top end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall...

, the entrance to the uterus, during childbirth

Childbirth

Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus...

, miscarriage

Miscarriage

Miscarriage or spontaneous abortion is the spontaneous end of a pregnancy at a stage where the embryo or fetus is incapable of surviving independently, generally defined in humans at prior to 20 weeks of gestation...

, induced abortion

Abortion

Abortion is defined as the termination of pregnancy by the removal or expulsion from the uterus of a fetus or embryo prior to viability. An abortion can occur spontaneously, in which case it is usually called a miscarriage, or it can be purposely induced...

, or gynecological surgery. Cervical dilation may occur naturally, or may be induced by medical means.
In the later stages of pregnancy, the cervix may already have opened up to 1-3 cm (or more in rarer circumstances), but during labor, repeated uterine contractions lead to further widening of the cervix to about 6 centimeters.