From Tranquil Spring:

Kiyoko has always envied her older sister Atsuko, who gets all the attention, and feels like an impossibly distant goal. A sudden change in Atsuko's life forces Kiyoko to re-evaluate their relationship, and her opinion of her sister.

The title of this series symbolizes the relationship between Kiyoko and Atsuko. It is taken from a mathematical construct, and can be thought of as meaning "vanishing point." This is the point at which, from the viewer's perspective, two parallel lines eventually meet. To illustrate, think of standing on railroad tracks that continue in a straight line as far as the eye can see; the tracks are parallel and will never actually meet, but from the viewer's perspective, they appear to get closer to each other the farther away they are, eventually meeting at the point beyond which they are no longer visible.

Source: https://www.mangaupdates.com/series.html?id=75960