A Homunculus (ホムンクルス Homunkurusu, Latin for "little man", plural: "homunculi"; from the masculine diminutive form of homo, "man") is a representation of a small human being. Popularized in sixteenth-century alchemy and nineteenth-century fiction, it has historically referred to the creation of a miniature, fully formed human. The concept has roots in preformationism as well as earlier folklore and alchemic traditions.
Homunculi in the Len'en Project
In the Len'en Project, the definition of homunculus has seemingly expanded to refer to all life created via alchemical means. A homunculus is born with all the knowledge in the world, and gradually loses it over time as they age. By the time they are fully grown, they would have lost a majority their knowledge, but would still be far wiser than the average human. Due to their inborn knowledge, all homunculi are also "naturals" when it comes to spell card making. Adagumo no Yaorochi's comments on Taira no Fumikado's appearance suggests that all homunculi have peculiar eyes, perhaps referring to the black dots on their irises.
Characters that are this species