Hypostasis comes from late Latin hypostăsis, in turn derived from the Greek hypostasis. The first definition mentioned by the Royal Spanish Academy (RAE) in your dictionary refers to take something abstract as real.
The hypostasis supposes consider an abstract concept and attribute an independent existence to it. In other words, it grants the rank of substance to what is just one relationship or one property of an element.
It is possible to associate hypostasis with idealism, since this philosophical movement supports the independent existence of ideas. The notion of hypostasis also appears in the field of Christian religion to name the people of the Holy Trinity.
In this religious case, the hypostasis is the singular substance of each of these three people (Dad, Child and Holy Spirit), who share an immaterial essence in common. Expressed in a different way, the Holy Trinity is made up of three hypostases.
The hypostatic union, in this framework, is the Incarnation. It is about the coexistence of the nature divine and the Human nature on Christ.
The legal Medicine, on the other hand, also resorts to the term. In this context, hypostases are the red spots that arise on the back of lifeless bodies due to the accumulation of blood.
The visceral hypostasis is one of the changes recorded in the body post mortem. This phenomenon occurs from the collection of blood in the lungs, intestines and other viscera, which acquire a reddish hue due to this particularity.