fellow student; fellow disciple
Refers to someone who studies under the same teacher or master, often in traditional arts, crafts, or martial arts contexts.
He is my fellow disciple, studying under the same master.
兄弟子 is a senior disciple, while 相弟子 is a peer disciple without implying seniority.
Compound of 相 (あい, mutual) and 弟子 (でし, disciple). The reading あいでし reflects rendaku voicing of でし.