不定期

He visits us at irregular intervals.

他定期地来探访我们。

Trees of the family Dipterocarpaceae dominate the canopy and emergent layers of the hyper-diverse, aseasonal, humid lowland tropical forests of Southeast Asia.

以龙脑物为主的东南亚热带雨林有一非常特别的现象，那就是定期的大开花现象。