noun. The act of entering, as into union or incorporation with something; a passing into or within.

noun. Act of entering; entrance.

noun. The act or process of entering or intruding

noun. The process by which a potentiality enters into actuality

noun. The inward migration of cells from the blastula during gastrulation

"Now our first impression is that at last we have come to the simple plain fact of where the object really is; and that the vaguer relation which I call ingression should not be muddled up with the relation of situation, as if including it as a particular case."