The term has been widely used in dating culture in the past few decades, although the concept of no strings attached sex is far from new.If you happen to have a purely sexual relationship with someone then it is considered an NSA relationship.No strings attached sex is about one thing - having sex without any emotional baggage.You simply meet up and have sex whenever you both want to and then part ways afterwards.NSA on the other hand, doesn't need to happen with a friend.
That's not to say you don't come across people in online dating that want FWB type relationships - some people are looking for friendship and sex without any of the baggage that comes with it!
With NSA, you aren't really friends so there isn't much to more to it than having sex.
With FWB, friendship comes first and the sex is just a nice added bonus. The more often you have sex and spend time with each other, the more chance there is of emotional attachment development, with most no strings attached arrangements ending after a few weeks or months or developing into a full-blown relationship.
This might be someone you casually hook up with from time to time or a one-night stand were you never took things further - it was and is only ever about the sex.
In the context of online dating, when someone is looking for NSA it means that they are only interested in sex and don't want to start dating or get serious, just have some no strings fun with a likeminded person. This is a type of relationship between two friends that happen to have sex with each other.