The antonyms for the term "external phase" would be "internal phase," "inner phase," or "interior phase." These terms refer to the internal part of a system or object, in contrast to the outer or surface part. For instance, in chemistry, the internal phase is the portion of a system that is dispersed in another material, such as an emulsion or suspension. In biological systems, the internal phase refers to the organs, tissues, and cells within an organism. Similarly, in architecture, the internal phase pertains to the layout, design, and functionality of the internal spaces within a building or structure. Therefore, determining the external and internal phase is essential in understanding the properties and behavior of a system.