Effect or efficacy (of a medicine, method, etc.); the impression or potency of something.
この薬は効き目が早く、飲んでからすぐ楽になった。
This medicine works quickly; I felt better soon after taking it.
化粧水の効き目を確かめるために一週間使い続けた。
I used the lotion for a week to check its effectiveness.
効き目が切れるとまた痛みが戻ってきた。
When the effect wore off, the pain returned.
民間療法の効き目には個人差がある。
The effectiveness of folk remedies varies from person to person.
One's dominant eye (the eye that is stronger or used preferentially when aiming/looking).
射撃では利き目を確認してから狙うことが大切だ。
In shooting, it's important to check your dominant eye before aiming.
友達は右利きだけど利き目は左だった。
My friend is right-handed, but their dominant eye is the left one.