noun
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'.
その紙上計画は現実的ではなかった。
That paper plan was not realistic.
紙上計画に過ぎないので、実際に試してみる必要がある。
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.