Behavioural biology is a major discipline within biology, centred on the key concept of `behaviour.' But how is `behaviour' defined, and how should it be defined? We outline what characteristics we believe a scientific definition should have, and why we think it important that a definition have these traits. We then examine the range of available published definitions for the word. Finding no consensus, we present survey responses from 174 members of three behaviour-focused scientific societies as to their understanding of the term. Here again, we find surprisingly widespread disagreement as to what qualifies as behaviour. Respondents contradict themselves, each other, and published definitions, indicating that they are using individually variable intuitive, rather than codified, meanings of `behaviour. ' We offer a new definition, based largely on survey responses: "Behaviour is the internally coordinated responses (actions or inactions) of whole living organisms (individuals or groups) to internal and/or external stimuli, excluding responses more easily understood as developmental changes." Finally, we discuss the usage, meanings and limitations of this definition. KeywordsBehaviour; Definition; Philosophy of Science; Levels of Organization The biological study of behaviour has grown tremendously over the last half century. Biosis Previews tags 42,286 items published in 2007 with the Concept Code "behavioral biology." Dozens of scientific societies, journals, courses, textbooks, etc. are organized around the central concept of behaviour. While behavioural biology interacts with a wide range of other disciplines, the unifying concept in behavioural biology is, as the name implies, behaviour. But while our understanding of behaviour has advanced tremendously since Tinbergen (1955) defined it as "the total movements made by the intact animal," our formal definition has failed to keep pace with this progress.What do we mean by this word, `behaviour'? There are numerous published definitions, and for many biologists the meaning is simply and clearly intuitive. However, satisfying definitions of this word, in the context of modern biology, are hard to find. Many definitions are so vague as to be impossible to apply. Others are crafted around a particular taxon such that members of other taxa by definition cannot behave (e.g. "Behavior involves the interaction between an Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author ManuscriptAnim Behav. Author manuscript; available in PMC 2010 July 1. Published in final edited form as:Anim Behav. Tinbergen...