Having sex, either oral or intercourse, with a random person, without any romantic involvement.
The chance meeting of two, who then proceed to engage in sexual relations, without any preliminary involvement, such as dating, or attempting to obtain anything more than that offered at the moment.
Chance encounters happen anywhere, at any time.
It is these encounters, that lead to both parties finding an attraction, that leads to sexual activity, without any expectations of more, that makes this act 'casual' in nature.
It can be where one party seeks out any willing partner, to engage in sex, which can be called a 'hook up' and nothing more.
This is the action of where neither party begins knowing the other, but develop sufficient friendliness, to agree to having sex.
Noteworthy: One Night Stand - where two meet, have sex, where at least one of the parties involved has no intention of seeking further involvement, or meetings with the other.
Hookup - Meeting another party, and engaging in a form of sex, which can be nothing more than manual stimulation, oral sex, or intercourse. The concept being that the meeting is unplanned.
Extended Hookup - Where a meeting occurs, but extends over a short period of time, again without any requested, or expected, emotional attachments.
Merely an extended time meeting, to have mutually agreed upon sex.
Anonymous Sex - Where both parties meet for the first time, and agree to having sex, and where BOTH parties have no expectation of seeking any further contact, or involvement.
Online Dating Sites or Contact Sites are classed as sites promoting casual sex, where unknown parties meet online, then meet in person to engage in sex, without emotional ties or involvement.
Some claim it is simply one step above Prostitution as the purpose of these 'dating sites' is to put together two people, for the purpose of having sex.
However, as money is not exchanged directly between the two parties, they are not illegal.