(Latin. Nisus: effort, striving, struggle. ) A term possibly introduced by Friedrich Casimir Medicus in the late 1700's (yet most known in relation with Blumenbach's Bildungstrieb) that was utilized by scientists investigating Vitalism to explain the impulse of nature to create forms and how those forms were managed. Darwin accepted this concept and discussed it in his writings, as did Immanual Kant.
"These causes [the secret force (vis occulta) responsible for the non-voluntary movements of the body] made me to follow up my own opinion, and to suppose a simple substance in addition to the organised matter and the soul; a substance that the creator gave to all organic bodies as a force bringing life into them. This force is in the vegetable and in the animal kingdoms the only force giving life to the organised matter. It is present in the man, too, where it causes all animal, or as other authors express it, all mechanic life. But man has a reasonable soul in addition to this organised matter and to this simple substance, the vital force; a soul that thinks and wants within him. Thus I think the man to consist of two simple substances, a soul and a vital force, and of a third one, of the organised matter.’ " - Medicus, lecture Von der Lebenskraft (On the vital force) 1774
The nisus formativus was therefore pointing towards the teachings of Aristotle, and what was already well known in esoteric mysticism, which is that physical dimension is a formation produced by the vital dimension, which in turn is a formation of the astral dimension, and so on into more and more subtle levels of nature. That is why the nisus formativus is variously described as pertaining to the vital body, the astral body, etc, because each one is the "formative force" of the more dense form below it. Each tradition represents this is various ways, such as the aggrgates of Buddhism, the koshas of Hinduism, etc. and are reflected in Kabbalah by the relationships between the sephiroth, specifically Malkulth (physical), Yesod (vital), and Hod (astral).
"To sustain it, the human body needs a nisus formativus — as Mr. Immanuel Kant, the philosopher of Königsberg said. That nisus formativus is the vital body or linga sarira of the Hindustani, the vital seat of any living cell, the Yesod [“foundation”] of the Hebrew Kabbalah. Just as our physical body has its vital body for its maintenance, sustenance, conservation, so likewise plants and any organism that has life; thus, in general, the whole planet Earth has it. So all the terrestrial world has its own vitality, vital foundation, its Yesod. Yes, in that Yesod of this terrestrial world [fourth coordinate] is the life of our world." - Samael Aun Weor, Alchemical Symbolism of the Nativity of Christ