From Volume 10.

Watakushi Space-Time (わたくし時空) is a term used by Amachi Hitsugi for Tenchi Academy. It summarizes Tenchi's status as her personal domain, where her private will is sovereign, all events proceed in accordance with it, and the ordinary laws of nature and rationality are without effect.

Watakushi ("I") is a highly formal first-person pronoun used exclusively (among the characters in the series) by Amachi, and serves as shorthand for her person. For a similar concept, see Watakushi is Rule.