In Dao De Jing, 常 should be translated as "eternal" or as "common"?
This article suggests that the original text was 道可道，非恒道. That is, 恒 not 常 (meaning 永恒 = eternal, unchanging).

This  dictionary definition for 常 also includes the meaning "长久，经久不变" (and gives the example 常量, with an alternate form 恒量).