Green wood is a term used to describe wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

 products that have been recently cut and have therefore not had an opportunity to "season" (dry) by evaporation

Evaporation

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

 of the internal moisture

Moisture

Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

. The term is used often in describing the relative moisture content of wood products such as firewood

Firewood

Firewood is any wood-like material that is gathered and used for fuel. Generally, firewood is not highly processed and is in some sort of recognizable log or branch form....

 and lumber. 'Green wood' being recently cut and high in moisture relative to 'seasoned wood' being lower in moisture because of drying due to the passage of time (seasons) or forced wood drying

Wood drying

Wood drying reduces the moisture content of wood before its use.There are two main reasons for drying wood:...

 as in kilns. Green wood is considered to be 100% moisture content relative to air dried or seasoned wood which is considered to be 20%. Available BTU

British thermal unit

The British thermal unit is a traditional unit of energy equal to about 1055 joules. It is approximately the amount of energy needed to heat of water, which is exactly one tenth of a UK gallon or about 0.1198 US gallons, from 39°F to 40°F...

 charts for wood fuels

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 tend to use air dried as their reference, thus oven

Oven

An oven is a thermally insulated chamber used for the heating, baking or drying of a substance. It is most commonly used for cooking. Kilns, and furnaces are special-purpose ovens...

 dried or 0% moisture content can reflect 103.4% BTU content, exceeding the mean value.
When green wood is used as fuel in appliances, it releases less heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 per unit of measure (usually cords or tons) due to the heat consumed to evaporate the moisture. The lower temperatures

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 resulting can lead to higher amounts of creosote

Creosote

Creosote is the portion of chemical products obtained by the distillation of a tar that remains heavier than water, notably useful for its anti-septic and preservative properties...

 being created which are later deposited on exhaust flues

Flue

A flue is a duct, pipe, or chimney for conveying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors. In the United States, they are also known as vents and for boilers as breeching for water heaters and modern furnaces...

. These deposits can later be ignited

Combustion

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

 when sufficient heat and oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 are present to cause a chimney fire

Chimney fire

A chimney fire is the combustion of residue deposits referred to as creosote, on the inner surfaces of chimney tiles, flue liners, stove pipes, etc. The process begins with the incomplete combustion of fuel in the attached appliance, usually a wood or coal stove...

 which can be destructive and dangerous.
'Green lumber' presents its own characteristics as well. Some species of wood are better utilized green due to less splitting when nailed green. Others tend to shrink excessively leaving voids between the individual pieces when allowed to dry. Often wood to be used for fine products such as furniture

Furniture

Furniture is the mass noun for the movable objects intended to support various human activities such as seating and sleeping in beds, to hold objects at a convenient height for work using horizontal surfaces above the ground, or to store things...

will be 'kiln dried' to stabilize it and reduce the shrinkage/expansion of the finished product.