What does loyalty in a relationship mean to women?

Well, different women are different, of course. I can only say that loyalty means everything to me.

It means remaining faithful to the parameters agreed upon for the relationship - and that will change depending on the relationship and the people who are in it.

Personally, I'm a woman who requires monogamy. So when I'm in a relationship, we are only sleeping with each other, only kissing each other and publicly declaring each other to be our one and only person with whom we have that sort of relationship. I hold myself to the same requirements.

Many women are like that and I would think assuming that would be a safe bet before you talk about it.

Some aren't. Some don't mind if their partner has sex outside the relationship (so long as they can too). Some are more stringent and see flirting and porn as cheating as well.

And then you get into the poly crowd whose rules are different again.

It really depends on you, your partner(s) and what you each need from a relationship.

Loyalty is subjective to the person you ask. Some women are less needy than others.

Simplest explanation I can think of (without going into trustworthiness, faithfulness, being supportive, etc) is you stand by your man or woman and don't just flake off randomly and abandon them because you see someone or something more interesting.