normal, natural, ordinary, regular, typical
All of these words are applied to that which is usual or expected because it conforms to a standard or rule. Normal implies that a particular person or thing does not exceed certain limits, or does not deviate far from an average or a standard established for a group, class, or species. [Normal body temperature ranges between 96.80 and 98.60 Fahrenheit; normal temperature in New York City for June is around 70o.
Regular implies accordance with some rule, plan, or method: to follow regular army procedures.
Typical indicates possession of those properties or characteristics that represent a particular class of things and differentiate it from all other classes of things: the typical clubbed antennae of the butterfly.
Ordinary, as an equivalent for normal or regular, emphasizes commonness or usualness and often stresses the absence of superior qualities. [An ordinary man is no genius;ordinary ways of doing things keep within the rules but do not initiate improvements.]
Natural emphasizes the agreement between an action and the innate character of the agent involved. Thus, it is natural for man to seek happiness, for birds to migrate, for the sun to set.
Antonyms: abnormal, atypical, irregular, queer, unnatural, unusual.
Ref: Hayakawa, S. I. (1982). Use the Right Word: Modern Guide to Synonyms and Related Words. Reader's Digest. 726 pp.