Translation

同然の / 等しい / 実質的に同じ

Meaning

Tantamount means equivalent in practical effect, even if not officially the same. It is almost always followed by to. The learner trap is confusing it with paramount, which means most important.

Examples

• その沈黙は罪を認めたも同然でした。
  His silence was tantamount to an admission of guilt.
• その提案を拒むことは、交渉を終わらせるのと同じでした。
  Rejecting the offer was tantamount to ending the negotiations.