Post-mortem interval is the time that has elapsed since a person has died

Death

Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

. If the time in question is not known, a number of medical/scientific techniques are used to determine it. This also can refer to the stage of decompostion the person is in.


Many types of changes to a body occur after death. Some of those that can be used to determine the post mortem interval are:

• Algor mortis
  Algor mortis
  Algor mortis is the reduction in body temperature following death. This is generally a steady decline until matching ambient temperature, although external factors can have a significant influence....
  
   - body cooling;
• Rigor mortis
  Rigor mortis
  Rigor mortis is one of the recognizable signs of death that is caused by a chemical change in the muscles after death, causing the limbs of the corpse to become stiff and difficult to move or manipulate...
  
   - stiffening of limbs;
• Forensic entomology
  Forensic entomology
  Forensic entomology is the application and study of insect and other arthropod biology to criminal matters. It is primarily associated with death investigations; however, it may also be used to detect drugs and poisons, determine the location of an incident, and find the presence and time of the...
  
   - insect activity on the corpse;
• Vitreous humour
  Vitreous humour
  The vitreous humour or vitreous humor is the clear gel that fills the space between the lens and the retina of the eyeball of humans and other vertebrates...
  
   changes - eye chemistry;
• State of decomposition - autolysis (process of self digestion) and putrefaction
  Putrefaction
  Putrefaction is one of seven stages in the decomposition of the body of a dead animal. It can be viewed, in broad terms, as the decomposition of proteins, in a process that results in the eventual breakdown of cohesion between tissues and the liquefaction of most organs.-Description:In terms of...
  
   (process caused by bacteria found within the body)

A person who judges the time of death by the means of decomposition is privy to a simple five stage process:

• Stage 1: Initial Decay - This is basically where the body will stop producing antigens and enzymes that are used to fight off bacteria located mainly in the lower intestine.
• Stage 2: Putrefaction - Because the body no longer has a defense system in place the bacteria grow and multiply by feeding off the body. They will begin to bring forth certain gases, which in turn will give the dead body a sort of bloated look and will cause a rather unpleasant odor.
• Stage 3: Black Putrefaction - This stage will bring further discoloration to the body (whether it be black, blue, purple, green etc.). It will also have an even more horrendous odor as the gases caused by the bacteria begin to escape out of the body.
• Stage 4: Butyric Fermentation - Where the internal organs begin to liquefy and the body will begin to desiccate-or dry out.
• Stage 5: Dry Rot
  Dry rot
  Dry rot refers to a type of wood decay caused by certain types of fungi, also known as True Dry Rot, that digests parts of the wood which give the wood strength and stiffness...
  
  - This is the slowest process out of the five stages. Basically, as the name indicates, the body will slowly begin to dry out and ultimately skeletonize.