That which is essential.
Also known as the true nature or simply the Truth.
Also known as Spirit.
That which is most important, not optional or changeable.
That which does not change.
That which is not name and form. That which is not false or illusory.
That without which the thing in question will not exist.
That which actually exists.
The substance. The ground. The fundamental being.
Consider a wooden table. Everything about the table, such as, its shape, color, size, components, design etc. can be changed, but not the wood itself. Without the wood the table won't exist. Here the wood is the essence of the table, or the nature of the wooden table, not the form or any other quality in the table.
Similarly, gold is the nature of gold ornaments, as it is the only one which does not change among changing forms of gold.
Clay is the essence or nature of all objects of the clay, not the names and forms it takes.
Water is the nature of waves.
On the Path Of Knowledge, knowing one's true nature is most important. This provides the answer to the question - Who am I?
Knowing one's nature is also called Self Realisation.