Is the period after mitosis, when the cell's cytoplasm completely divides, called interphase or cytokinesis?
The answer to your question is cytokinesis. It is important to remember mitosis only involves the chromosomes and structures within the nucleus of the cell. When the chromosome replication and division is complete, with two nuclei in the cell, the process of cytokinesis starts. I teach my students the prefix "cyto-" means "cell", while the suffix "-kinesis" means "motion". So cytokinesis literally means "cell motion", where the cell membrane parts down the middle, separating the cytoplasm, the two nuclei, and all the other cell parts into two separate cells. Interphase is the period of time the cell spends conducting its other life processes. The majority of the cells time is spent in interphase, with mitosis and cytokinesis occupying a smaller percentage of time. A good way to remember the order of these processes is interphase occurs first, with mitosis in the middle, with cytokinesis finishing third.
Mitotic cell division produces two daughter cells that are genetically identically to the parent cell only smaller. That is because once the chromosomes replicate and undergo mitosis, the cytoplasm divides and the cell membrane pinches in the case of animal cells into two daughter cells. In plant cells, a cell plate forms and separates the two newly formed cells. The answer to your question is cytokinesis. Both mitosis and cytokinesis make up the M phase of the cell cycle. Interphase is the resting phase when the cell grows and replicates its chromosomes before beginning another round of mitotic cell division. Cells spend most of their time in interphase. Interphase is the G1, S, and G2 parts of the cell cycle.