ruling at the present time or at a particular time, or relating to the period when a king, queen, etc. is ruling

Mary, Queen of Scots was Scottish queen regnant from 14 December 1542 to 24 July 1567. 苏格兰女王玛丽一世于1542年12月14日至1567年7月24日期间在位。

I did hear once that Prince Charles is considering taking as his regnant name as King George VII. 我曾经听说查尔斯王子考虑在继承王位以后使用乔治七世这个称谓。

having the most influence at the present time or at a particular time

That is the ideology that is regnant in France. 那就是在法国占据主导地位的意识形态。

Failure to meet the psychological needs of the developing child can be a regnant cause of personality disorders. 没能满足儿童在成长过程中的心理需求是造成他们性格缺陷的主因。