If i am right, then any category placed at any rank in the taxonomical hierarchy is a taxon. For example Panthera, Mammalia, Solanaceae etc. But are general group of organism (casual groups which are not biologically defined like Birds, Cats, Fishes) also regarded as taxa? Please clarify.
Definition of taxon (From ICZN glossary):
A taxonomic unit, whether named or not: i.e. a population, or group of populations of organisms which are usually inferred to be phylogenetically related and which have characters in common which differentiate (q.v.) the unit (e.g. a geographic population, a genus, a family, an order) from other such units. A taxon encompasses all included taxa of lower rank (q.v.) and individual organisms. The Code fully regulates the names of taxa only between and including the ranks of superfamily and subspecies.
Birds are called Aves and that is a taxon. Cats come under Felidae. If you are asking - "Is a colloquial/vernacular name considered a taxon". Perhaps the group of organisms can be referred to by a colloquial name when they are not given any scientific name but in this case these taxa are well defined by a standard scientific name (according to the nomenclature codes such as ICBN, ICZN, etc).
So "cat" is not a taxon, but cats fall under the taxon called "Felidae".
In some cases certain groups are not even actual clades. A clade is a monophyletic group i.e. all its members have a common ancestor. The group — reptiles, is not a clade.