paper plan; plan on paper; theoretical plan
Refers to a plan that exists only in theory or on paper, often implying it has not been tested in practice. Similar to 'paper plan' or 'armchair strategy'.
That paper plan was not realistic.
It's just a plan on paper, so we need to actually try it out.
机上の空論 is a more common idiom for 'armchair theory' or 'impractical idea', while 紙上計画 specifically emphasizes a plan that is written down but not yet executed.
Compound of 紙上 (on paper) and 計画 (plan). The exact historical derivation is uncertain, but the meaning is transparent from its components.