No satisfactory definition of a neoplasm exists. Current criteria are shown to be invalid for strictly definitional purposes. Difficulties relate in part to the difference between descriptions and explanations, and also to the status of definitional "truths". Definitions function to assist recognition, convey meaning and highlight the essence of what requires further explanation. Existing deficiencies and the provision of an adequate alternative are therefore matters of interdisciplinary importance with broad ramifications, particularly for the design of rational therapy. A new definition is proposed which, while saying as much as can be said, is conservative. Criticism of its logical base is anticipated and countered; two possible problems cases are admitted and discussed.