Many people mis-understand and mis-define “gasification”, “producer gas”, “pyrolysis” and “syngas”. For example:
The UK Government Department of Energy and Climate Change define “syngas” (incorrectly) as “produced by either pyrolysis, indirect gasification or oxygen blown gasification of solid waste or biomass. Syngas is distinct from producer gas, which is produced by air blown gasification of solid waste or biomass. Syngas has a higher calorific value than producer gas, owing to the absence of nitrogen, which is present in producer gas.” 
Here is an alternative definition from a very recent academic text book (again not wholly accurate) : “At intermediate temperatures typically between 500 and 700°C and in the absence of such reactants [oxygen, hydrogen or water], the process is called pyrolysis. If O2 is added (with or without other educts) and at higher temperatures (> 800°C), pyrolysis turns into gasification. With increasing amount [sic] of O2, gasification converts to combustion (at high temperatures).”
Because of the confusion, others choose to create their own definition : “And we will use the term syngas to describe producer gas which has been cleaned sufficiently so as to be suitable for gas engines and turbines”.
The International Energy Agency give a better definition : “Gasification is a process in which a solid fuel (e.g. coal, biomass, wastes) is converted into a combustible gas, called producer gas or syngas. Gasification takes place at high temperatures (700-1500°C), and heat or small amounts of air or oxygen are added to supply the energy needed for the gasification process”.
Gasification and pyrolysis are old technologies, and these terms for both the systems and the gases that they produce relate back to their origins, 100+ years ago. Because gasification and pyrolysis now have greater focus of attention because of their potential to convert waste into energy, people, including government agencies are applying these terms without knowing what they mean. Here is a correct definition of the terms:
“Pyrolysis” is unoxygenated heating of a solid fuel. “pyro-” = fire or heat,”-olysis” = breaking up/decomposing.
“Gasification”, is simply, as its name suggests “gas production”. Pyrolysis occurs as part of the gasification process. In fact all the gases originate from the pyrolysis chemical reactions. However a reactor designed for gasification (a “gasifier”), encourages other chemical reactions inside it (see http://blushfulearth.co.uk/gasificationpyrolysisact/). So, gasification is not defined by temperature, or the amount of oxygen, steam or air, that is allowed to enter the reactor.
Combustion is thermal decomposition in the presence of oxygen (or air, since air contains 21% oxygen). Gasifiers always allow some oxygen (or air or steam) to enter and by doing this they burn (by combustion) a fraction of the fuel to provide energy to keep the gasification process going. The optimum is usually 25% of the oxygen necessary for full combustion.
“Producer Gas” is the correct name for gas from a gasifier. It comes from the fact that historically, gasifiers were (and still are) “gas producers”, and some people also think that the name relates to the fact that as soon as the gas was produced it was used .
“Syngas” is a gas produced by chemical synthesis – e.g. formed from separate elements – the origin of its name, which is shortened from “synthesis gas”. It is not shortened from “synthetic gas”. Synthesis gas has carbon monoxide and hydrogen as its main components, along with lesser amounts of carbon dioxide and methane, all of which usually are diluted by inert nitrogen. Now, “producer gas” has the same composition as “synthesis gas”, hence why the two terms are being mistakenly used as synonyms. Confusing matters though is the fact that “producer gas” can be used for chemical processing (although most syngas is produced by the steam reforming of methane). But, by definition, if the producer gas from a gasifier is burned in an engine or turbine, then it should not be called “syngas”.
 UK Government, Department of Energy and Climate Change, RHI Evidence Report: Direct application of renewable heat, 29th October 2014, DECC: London.
 Bucheli, T.D., Hilber, I., Schmidt, H-P. Polycyclic aromatic hydrocarbons and polychlorinated aromatic compounds in biochar. In: Lehman, J., Joseph, S. Biochar for Environmental Management, science, technology and implementation (2nd Ed.), (2015), London: Earthscan, pp. 595-624.
 Ernsting, A. Biomass gasification & pyrolysis, (2015), Biofuelwatch.
 International Energy Agency. The Task 33, Thermal Gasification of Biomass. What is gasification?
 Generator Gas – The Swedish Experience from 1939 – 1945, The Swedish Academy of Engineering Sciences, SERI/SP-33-140, (1979), Solar Energy Research Institute: Colorado.