Whether the halfway word is defined on English dictionaries as an adverb/adjective mostly used to indicate that someone or something is At or to a point equidistant between two others, I'm usually drawn to sentences such as the following: I'm half way finishing the translation. I'm half way in to the second season. I'm half way there.
Technically, translucent is not halfway between transparent and opaque. Both translucent and transparent pass light; opaque does not. A 50% translucent object would pass the same amount of light as a 50% transparent one. Only in ordinary speech is translucent considered a synonym of semi-transparent.
0 Depending on the use (eg if it is in literature), the phrases "up/down through the stairs" is mostly likely to naturally convey of an object/person in this position. For example: "halfway down the stairs, she turned and looked back at the man on the floor above her".
1 How about extent? The extent of the class is four students. The extent of the container's contents is halfway. The extent of the battery's charge is halfway. The extent of the progress is low.
Given: semi-, prefix: half of or occurring halfway through (a specified period of time) [MW] A semi-anniversary (or semianniversary) would be a six-month anniversary. (Edit: The term di-anniversary shows up in searches; the only problem is, di- means "twice : twofold : double" not "half.") Some Googling turns up mensiversary (from Latin mensis for month) as a term for a one-month anniversary ...
When talking, this is usually referred to as " trailing off ", or "trailing off in the middle of a sentence". Eg "Susan started telling John about the weekly sales report, then trailed off, looking over his shoulder." "I'm always trailing off halfway through a sentence, especially when I haven't had a coffee yet." Alternately, the trailing off describes' someone's voice: "Susan started telling ...