limen

Limen, liminis, n., third declension. The stem is limin-, so the genitive plural is liminum and the nominative and accusative are the same shape, as they always are in a neuter. Keep it apart from limes, limitis, m., which is a boundary or a frontier road: the two words look like relatives and behave like strangers, and a limes crossed by an army is not a limen crossed by a guest.

Medieval usage gives it a second life. A journey ad limina apostolorum, "to the thresholds of the apostles", is a pilgrimage to the tombs of Peter and Paul in Rome, and the bishops' obligation to make it is still called the visit ad limina. English keeps the word inside eliminate, which is ex plus limen, to put out of doors, and inside preliminary, the thing done before you are over the sill.