observant

Observant means quick to notice things — or careful to keep religious rules. Two senses that sound unrelated and are not.

The noticing sense is a compliment, and describing somebody as observant means they see what others miss, which is what detectives and good doctors are praised for. The religious sense means keeping the practices of a faith, so an observant Muslim or an observant Jew keeps the required observances. The link is attention, since both mean watching carefully what should be watched. The noun observation belongs to the first sense and observance to the second.