Cuckolding is the process of the male half of a partnership not being able to sexually perform (whether the cause is mental or physical), and the sexual needs of the female half of the relationship (whether wife or or not, could be a girlfriend) are taken care of by a surrogate male, usually a Dom. Cuckolding is basically a humiliation process, and cuckold Doms often have sex with the female, in front of the incapable male partner, and the incapable male partner is often responsible for creampie cleanup.
Cuckolding is more prevalent in relationships where theer is a huge disparity in the ages of the male and female. When a 20 year old female marries or hooks up with an older man (often a wealthy older man), there usually comes a point in time that the older male can't get it up anymore, and the needs of the female are addressed by a cuckold Dom who sexually services the female, usually in the presence of the incapable older male. This is a lifestyle preference to mere adultry, wher the female gets her sex when and where she can find it... on her own. Cuckolding is distinguished by the presence and obvious humuliation of the older male, who often pays for the cuckolding... and then cleans up the creampie.
It is more prevalent in age disparity couples, as not too many cuckold Doms are eager to go have sex with a 78 year old woman in front of her 79 year old husband/mate. It is more often like a 79 year old man with a 20ish-30ish-40ish year old wife/mate. Cuckolding more commonly involves attractive and younger females, being sexually serviced in front of their elderly (or mentally/physically incapable) mates/husbands/boyfriends.
Cuckolding is lifestyle related, and sometimes may not involve sex, but usually involves acts of submission by the female half... to a Cuckold Dom. I don't know what the Domme equivalent of cuckolding is, but I'm sure it exists via incapable or disinterested female halves, as a measure to ensure submission or sex for the other half, the male. Cuckolding is not simply infidelity, or screwing around... it usually involves humiliation.
As far as King Arthur, I'd say it was Gwenivere who chose the affairs with Lancelot, and not Arthur. Also, I think Lancelot/Gwenivere liaisons were essentially covert, the sex was done in private, as they could steal away... and not in front of Arthur, and the King subsequently performing creampie cleanup. Their "affair" was just that... an affair, but a long lived affair, and not necessarily a good example of the lifestyle concept of cuckolding, or it's application in general lifestyle terms.
Modern day dilution and cross application of the lifestyle term has produced almost slang derivatives and many use cockold as a buzzword. The Urban Dictionary lists several user added definitions, and in reading them you can see many modern day slang adaptations of the buzzword.
In addition, I've always been of the impression cuckolding was not based simply on an incapability of the male partner... cuckolding can be a punishment for him as the female partner chooses, while he may still be quite capable... but be denied for whatever reason and the male may be forced to watch another male with his partner.
Hope this helps to put cuckolding in a better perspective.
Careful with that axe, Eugene