n.
Commonly, any insect ; scientifically, any member of the insect order Heteroptera.
In scientific usage, when the word "bug" is part of the common name for a member of the order Heteroptera, it is a separate word (e.g., "chinch bug"); when used as part of the common name for an organism that is not a heteropteran, it is not separated (e.g., the ladybug, in the order Coleoptera). In common usage, there are many exceptions to this convention (e.g., bedbugs are heteropterans).