Not all marriages have a happy ending, and divorce among spouses is common these days.
While some partners call it quits and split up, many will go on to find new love and even remarry. If they end up joining a family with children, they will take on the role of step-parent.
A step-parent, including step-dad and step-mum, is a person who marries a someone after the divorce or death of their partner, but has no biological relationship to their child.
The prefix 'step' has been used for more than 1,000 years - but people are only just learning what it actually means.
A statement on Bonus Families reads: "The prefix 'step'- used in this sense is Middle English, derived from Old English steop.
"The Old English form comes from steopcild ('stepchild'), which meant 'orphan'.
"The steop- prefix comes from Old English astiepan/bestiepan 'bereave' (with cognates in Old High German arstiufen/bestiufen).
"The sense is that an orphan is bereaving his lost parent(s).
"Before 800, stepfather/stepmother meant 'one who becomes a father/mother to an orphan', and stepson/stepdaughter meant 'an orphan who becomes a son/daughter by the remarriage of a parent'."
However as divorce became more common, the word evolved to include those who choose to separate, meaning "death was not required in order for a stepfamily to be born".
Commenting on this revelation, one Reddit user said: "It's a weird term that doesn't really make sense, like mother-in-law.
"My husband's mum is not my mum any more than I am my step kid's parent, but it's just a word we got used to using."
Another user added: "Stepparent is the absolute worst word ever invented - stepparents are not parents."
A third user said: "In my experience it seems it's cause the kids try to step all over you. Because you're not my dad, I'll do what I want. In the mothers eye's, they're little angels."
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