What is Shikantaza?

If I tell you to sit and meditate, what do you do?
In a recent question the person asked what to do when losing focus in shikantaza: return to the breath, return to body awareness or otherwise?
If you were doing 'sit and meditate' these would be valid answers.
However, shikantaza is not 'sit and meditate'. It's Just, only, exactly, precisely sitting.
The moment you return to the breath you are not doing 'only sitting'.
The same applies to 'return to awareness of the body', this is also not 'only sitting'. The moment you are thinking about sitting you are not doing Just, only, exactly, precisely sitting. Even when you sit without any point of focus but sitting and have the notion 'I'm doing 'sitting and nothing else' you are off.

To know the difference you'd have to experiment. What does it mean to sit and meditate? You probably know this. 
But what does it mean to do Just, only, exactly, precisely sitting?
You'd have to work to understand just that. And it's not an easy task.
Quora - Internet.- The difference between 'just sitting' and meditation.