(Note, much of the following relates to the industrial drying of sawn, square-edged timber, but the principles involved are also appropriate to the drying of wood in other forms).
The term "seasoned" is a commonly used term for air dried timber.
At its simplest air seasoning of timber involves constructing stacks of sawn wood and leaving the stock to dry making use of the wind and the sun.
We have only limited control over the effects of the wind and the sun and this is effected via the correct placement and construction of the stack to be dried.
In Europe (though not in all countries in the world) we generally try to accelerate the drying process.
In Europe drying times are long. Approximate drying times are:-
When the air seasoning of softwood timber is commenced in the spring season (so drying can occur over the summer season, as softwood species, being more permeable than hardwood species, can tolerate 'rapid' drying), times of drying can be:
When the air seasoning of hardwood timber is commenced in the autumn season (the air seasoning of hardwoods being best undertaked during the cooler seasons - as hardwood species are less permeable than softwood species and therfore cannot tolerate 'rapid' drying), times of drying can be:
For an introduction to the permeability of wood see The Structures of Softwoods and Hardwoods and their effect on Wood Drying.
At one time it was believed that timber that had been air dried was of superior quality to timber which had been dried in a kiln - there is in fact little evidence to support this idea other than the benefit of the more gradual dissipation of drying stresses which can occur when good quality air dried wood is stored properly and moisture gradients across the timber cross section allowed to even out (such stresses in dried timber can be deleterious and cause a number of defects in the dried timber).
However a serious limitation of air seasoning of timber in UK (and similar climates) is the minimum moisture content attainable, due to the prevailing climate. In winter only as low as 23% can be attained, whilst even in summer 17% is the lowest moisture content that can be obtained. These values are NOT LOW ENOUGH FOR MOST INDOOR APPLICATIONS. So in Europe we must also kiln dry for indoor applications.
Now Read about the Best Practices when Drying Timber in the Drying Yard.