Longman Dictionary of Contemporary English
Wiktionary
n. A parent who is the only guardian of an offspring.
Wikipedia
A single parent is an uncoupled individual who shoulders most or all of the day-to-day responsibilities for raising a child or children. A mother is more often the primary caregiver in a single-parent family structure that has arisen due to death of the partner, divorce or unplanned pregnancy.
Historically, death of a partner was a major cause of single parenting. Single parenting can also result from the breakup or divorce of coupled parents. Custody battles, awarded by the court or rationalized in other terms, determine who the child will spend majority of their time with. In western society in general, following the separation of a heterosexual couple, a child is placed with the primary caregiver, usually the mother, while the secondary caregiver is usually the father., though the reverse does happen and joint custody is on the rise.
Fathers have been the less common primary caregiver in the recent past, presumably due to the father working most of the day resulting in less bonding with the children, or possibly a young child needing to still nurse, or if childcare was necessary while the father works, the mother would be seen to be better suited while fathers works. This scenario has shifted in recent years, as many fathers are taking an active parental role as a stay-at-home dad as more mothers are in the workforce and being the sole provider to the family, resulting in fathers bonding and connecting more to their children.
Recent years have seen the increasing incidence and visibility of uncoupled women who choose to be single parents. When single women seek to get pregnant intentionally in order to become single mothers by choice (or "choice moms"), they often seek an anonymous or known sperm donor. Single parent adoption or fostering is also sometimes an option for single adults who want to raise a family.
The demographics of single parenting show a general increase worldwide in children living in single parent homes. Although divorce is one of the main events that leads to single parenting, it may be that the majority of cases in the US are from pregnancy outside of wedlock. Single parenting has become a norm in the United States and is a trend found in multiple other countries. The morality and advisability of single motherhood has long been debated in the US; the topic is less contentious in Western European countries where all families enjoy more robust state-sponsored social benefits.