Fundamentally, temperature is the potential for heat transfer. Thus an object at a higher temperature will lose heat to an object at a lower temperature when they are placed in contact. We all know what we mean when we talk of 'the temperature' of an object: it is just a measure of how hot or cold it feels. But our sense of temperature is qualitative rather than quantitative. Engineers and scientists can routinely create temperatures from very close to absolute zero, up to a few thousand degrees. To understand what we mean by temperature when we are dealing with situations well outside our experience we need a more specific definition of temperature and a reliable numerical scale. The understanding of how this can be done came with the development of thermodynamics and statistical mechanics in the nineteenth century. One important deduction can be stated as:

The temperature of a substance (or of an object) is a measure of the average energy of the microscopic objects of which that substance is made.

However, one or two points need explaining further.

• The "microscopic objects of which the substance is made" are usually the atoms or molecules. However, if the measurement being made is of the temperature of a blackbody, then the microscopic objects are the photons which make up the electromagnetic field within the blackbody cavity.
• The "average energy" is slightly loose terminology, but is a fair description for many purposes.
• The "temperature" refers to the absolute temperature which is measured in units of the kelvin (K). The temperature of a substance at 0 ºC is actually 273.15 K which is clearly far from zero, indicating that the atoms and molecules of a substance at 0 ºC still have a great deal of energy.

Links: You can find lots more background information about temperature at http://www.unidata.ucar.edu/staff/blynds/tmp.html.