Miscible is a fancy word for "mixable." You may have heard that oil and water are not very miscible substances, whereas seltzer and orange juice are miscible and delicious!
Miscible is a word used by chemists to explain why some liquids mix together well, while others do not. Miscible liquids form homogeneous solutions, which is another way of saying liquids that have a uniform nature. Liquids that are not miscible will not form a new liquid; instead, they will each rest separately in a container (like oil and water). Miscible can also refer to solids and gases, although such use of the word is rare.