In common usage, a butter knife may refer to any non-serrated table knife designed with a dull edge and rounded point; formal cutlery patterns make a distinction between such a place knife (or table knife) and a butter knife.
In this usage, a butter knife (or master butter knife) is a sharp-pointed, dull-edged knife, often with a sabre shape, used only to serve out pats of butter from a central butter dish to individual diners' plates.
Rather, diners at the breakfast, the luncheon, and the informal dinner table use an individual butter knife to apply butter to their bread.
[1] Individual butter knives have a round point, so as not to tear the bread, and are sometimes termed butter spreaders.
If no butter spreaders are provided, a dinner knife may be used as an alternative.