persistence persistency

Both words have the same meaning and they remain largely interchangeable, although in current use persistence is commoner. You won’t find “persistency” in most British dictionaries.

If you have persistence (坚持, 毅力), you continue to do something even though it is difficult or other people are against it.

“Only agelessly silent, with a reptile's persistency” — quote from the poem “Tortoise Gallantry” ~ by D. H. Lawrence, 1921