The crossword solution for the term 'highly observant' is 'PIOUS'. PIOUS is an adjective that describes someone who is deeply religious and devoted to their faith. It is often used to describe someone who is very conscious of their moral obligations and follows them closely. A highly observant person is someone who pays close attention to their surroundings and notices even the smallest details. They are often very aware of their environment and can pick up on subtle changes or nuances.