The sequence of changes over the full lifespan of an organism. This is the meaning first introduced into psychology; the area of ‘developmental’ psychology in the early decades of the 20th century referred to the study of the full lifespan, from birth to death. Today the tendency is to use the term more restrictively. See, for example, DEVELOPMENTAL DISABILITY and DEVELOPMENTAL *APHASIA, in which the age range is restricted to birth to adolescence.
Maturation. The connotation here is that the process is a biological one and largely dictated by genetic processes. This sense is probably the oldest and etymologically goes back to the Old French desveloper meaning to unwrap or unfold. Developmental processes of this kind are often contrasted with those that are the result of learning. See here the discussion under CHILD DEVELOPMENT.
An irreversible sequence of change. To some degree this notion of irreversibility is contained in meanings 1 and 2, but it is listed separately because of its significance in medicine and psychiatry, which refer to the developmental course of a disease or disorder, over which a number of distinct stages follow one upon the other.
A progressive change leading to higher levels of differentiation and organization. Here the connotation is one of positive progress, with increases in effectiveness of function, maturity, sophistication, richness and complexity. This sense is generally intended in phrases like human development, social development, intellectual development and emotional development. Note that the genetic connotations of meanings 2 and 3 are absent here; rather, the implication is that processes attributable to environmental factors (learning, nutrition, etc.) are responsible.
Clearly, we have a rather loose term on our hands here. And, as is so often the case with terms that reference processes of fundamental importance, it is applied very broadly. In almost any of the above senses, that which develops may be just about anything: molecular systems, bones and organs, emotions, ideas and cognitive processes, moral systems, personalities, relationships, groups, societies or cultures. Not surprisingly, there is a large number of specialized terms based on this one; the more commonly used follow.