The main purpose of the paper is to discuss the advantages and drawbacks of the great number of conditions revealed by the literature for characterizing biomass fast pyrolysis. They are often related to temperature and heating rate. Unfortunately, significant inconsistencies exist between the values recommended by the authors. In order to bring new insights to this problem, a very simple model of solid pyrolysis is solved in the case of global primary cellulose pyrolysis. The operating conditions are chosen in order to cover almost all practical situations. The two cases of fixed heat source temperature and of temperature increasing with time are considered. The results show that actual particle temperature and heating rate are inappropriate for defining fast pyrolysis. Other criteria are also discussed. Among them, reactor temperature and external heat transfer coefficient appear to be more relevant, as well as efficiency of primary products removal. However, it is shown that the definition of fast pyrolysis reactor optimal conditions cannot rely on only one criterion. All implied parameters should be taken into account in a global approach including close couplings between reactor properties and particle pyrolysis behaviour.