Think of trying the dictionary?

From Webster.com:

Main Entry: se·dan
Pronunciation: si-'dan
Function: noun
Etymology: origin unknown
1 : a portable often covered chair that is designed to carry one person and
that is borne on poles by two people
2 a : a 2- or 4-door automobile seating four or more persons and usually
having a permanent top

From http://www.autocenter.com/about/:
sedan
"sedan" refers to a fixed-roof car with at least four doors or any
fixed-roof two-door car with at least 33 cubic feet of rear interior volume,
according to measurements based on SAE standard J11OO.

http://en.wikipedia.org/wiki/Sedan

A sedan seats four or more and has a fixed roof that is full-height up to
the rear window. Most commonly it is a four-door; two-door is rarer but they
do occur (more so historically). In the U.S., this term has been used to
denote a car with fixed window frames, as opposed to the hardtop style where
the sash, if any, winds down with the glass. As hardtops have become rarer,
this distinction is no longer so important.

When I was youngs (60's), if a car has a B pillar, it was called a sedan. If
there was no B pillar it was a hardtop, or possibly a coupe. But that is not
a hard and fast rule. Ford call the current Mustang a coupe but it has a B
pillar of sotrs (although the window behind it is fixed) since there are no
frames around the passenger's and driver's door windows, I suppose the coupe
term is applicable. When I was really young, full size cars were ofen
offered as 2 and 4 door sedans and 2 and 4 door hardtops. The sedans had a
fixed pillar between the front and rear windows, the hard tops did not. A
coupe would be the same as a 2 door hardtop, except there would not be a
rear window (and hence no real B-pillar).

Ed

See Dan, See Dan Run......... :-)