another mountain; another temple
Rare standalone noun. Almost always encountered in the proverb 他山の石 (a lesson drawn from others' mistakes). The literal meaning 'another mountain/temple' is mainly a dictionary gloss; modern usage is overwhelmingly bound to the proverb.
The proverb 他山の石 means that even others' mistakes can serve as a lesson for oneself.
lesson from others' mistakes; food for thought
From Chinese 他山 (tāshān, 'another mountain'). The full proverb 他山之石 (tāshān zhī shí) appears in the Classic of Poetry, meaning stones from another mountain can polish one's own jade—i.e., others' faults can refine one's own character.