一いっ過か性せい

noun which may take the genitive case particle 'no', noun

transient; temporary; passing; fleeting

Often used in medical or technical contexts for symptoms or phenomena that appear briefly and then disappear. Can also describe general short-lived states.

• この症しょう状じょうは一いっ過か性せいのもので、すぐに治おさまります。

  This symptom is transient and will subside quickly.

• 一いっ過か性せいの流りゅう行こうに過すぎないと思おもう。

  I think it's just a passing fad.