Literally: one – eye – clear – “in that way”
Alternately: Clear (in the sense of “easily understood” rather than literally transparent); apparent; obvious. Able to be understood after a mere glance.
Notes: It is also acceptable to write this as 一目了然. The meaning and pronunciation do not change, but a simpler character is substituted in for the relatively rare and complex 瞭.
This compound comes from the 朱子語類 (in Japanese, shushigorui; in Chinese, apparently zhu ai yu le), a Confucian text from 1270 CE, compiled into 140 volumes. Ichi.moku.ryou.zen is from the 137th.