Holiness - toward a definition

So what is it?

Holiness I mean.

We say that there is a universal call to holiness. We talk about holiness of life? We say that religious sisters, in particular, are called to live a holy life.

The way of perfection.

Saints are holy.

God is holy.

What, exactly, are we talking about?

I am spending a lot of time these days, wandering around, asking people this question. In following posts, I'll share a synthesis of the answers.