The property or set of properties that make something what it fundamentally is. Hence if you remove an essential property, the thing ceases to be what it is.
For example, an essential property of a triangle is that it has 3 sides. If you add a side, it ceases to be a triangle and becomes a rectangle.
This is in contrast to accident: a property that the entity or substance might or might not have, i.e., if you remove an accidental property the thing does not cease to be what it is.
For example, if you change the colour of the blue triangle to red, it is still a triangle.
1. The concept originates with Aristotle who called it ti en einai, the what it was to be.
2. The corresponding scholastic term is quiddity.