The terms "agglomerate" and "aggregate" are widely used by powder technologists to describe assemblages of particles that are found in dry powders and powders in liquid suspensions. Each term has a specific meaning but, unfortunately, they are frequently interchanged at will and this has resulted in universal confusion. This confusion is perpetuated by conflicting definitions in national and international standards and this presents problems when describing powder properties or communicating results in reports and research papers. This paper reviews the current status of the definitions, with particular emphasis on their use in the pharmaceutical industry. It is proposed that just one term, agglomerate, should be used to describe an assemblage of particles in a powder and that the term aggregate should be confined to pre-nucleation structures.