nvl() function

I am trying to find the equivalent in Postgresql to the Oracle sql function
nvl(). With nvl() you give two parameters. The first may be a field/column
or variable. If the value is not null then it is returned by the function.
For example the with query below if the :ClientParameter is passed then only
those rows which have a clientdesc matching the parameter are returned. If
the :ClientParameter is null then those rows which have clientdesc =
clientdesc are returned (all rows):

select clientid,
clientdesc
from clients
where ( clientdesc = nvl( :ClientParameter, clientdesc ) )

I have looked thru all the documentation I could find but nowhere were any
built-in SQL functions delineated. Does anyone know where the documentation
can be found?

Thanks,
Steve