An inchoation is the very first spark or earliest stage of something. A seedling is an inchoation of a tree: It's a start, but it's not yet a tree.
The word inchoation is in the same family as inchoate, which describes something that is only partly in existence, as of yet imperfectly formed, such as the spark of an idea. Similarly, saying that something is an inchoation implies that there is more to come, like a rough sketch of a masterpiece. In legal terms, an inchoation is the point in time when some act begins, such as early ideas or discussions of committing a crime: The crime hasn't happened yet, but it's being planned.