Seasoned Firewood is dried wood, wood whose moisture content is less than 40% (ideally 20-30%).  Wood doesn’t lose much moisture through the bark, so splitting it is the most effective drying method.  Moisture is efficiently removed through the cut cells at the end of each piece.  Wood should dry from 9months to 2 years for best burning (highest heat/BTU output).  Signs of properly seasoned firewood include: checks (cracks) in the ends that radiate out from the heart like hands on a clock, darkened ends, loss of wood color, lighter wood, loss of bark, no signs of mold/fungus, lack of smell and a loud "clang" when banged together end to end (like a baseball bat).