In situ means "in place". It applies to epithelial tissue only (no such thing as "sarcoma in situ"). The tumor must be microscopically diagnosed in order to visualize the basement membrane. If the tumor is shown to be microinvasive, case is considered localized.
non- or pre-invasive, non-infiltrating, intra-epithelial, Stage 0, intraductal, intracystic, no stromal invasion, no penetration below the basement membrane.