There are two questions to be considered:  one is linguistic, the 
other historical.  Etymologically, I believe that previous contributors 
are correct in asserting that stauros is so general a term as to be 
applicable to almost any simple wooden implement -- a rail, fence post or 
gate, for example.  Historically, two things may be said.  1) In the 
specific biblical case, we must consider Roman usage.  (Jesus was, after 
all, put to death by the Romans.)  The Roman implement for death by 
torture was call "crux" in Latin.  This term unambiguously means 
something which crosses (in T, X, or traditional "cross" shape).  2) 
Further, the Roman writers (including non- and pre- Christians) used 
"crux" as a translation of stauros, just as their Greek counterparts used 
stauros as a translation of crux, implying a pragmatic equality of 
concepts.  In the 1st century, a stauros (when the term is used of the 
instrument of execution) was a wooden cross, though not necessarily in 
the traditional form.