The term negiah (Hebrew: נגיעה), literally "touch," is the concept in Halakha that forbids or restricts physical contact with a member of the opposite sex (except for one's spouse, children, siblings, grandchildren, parents, and grandparents). A person who abides by this halakha is colloquially described as a shomer negiah ("one observant of negiah").
The laws of negiah are typically followed by Orthodox Jews, with varying levels of observance. Some Orthodox Jews follow the laws with strict modesty and take measures to avoid accidental contact, such as avoiding sitting next to a member of the opposite sex on a bus, airplane, or other similar seating situation. Others are more lenient, only avoiding purposeful contact. Adherents of Conservative and Reform Judaism do not usually follow these laws.
Read more about Negiah: Biblical Prohibition and Subsequent Exegesis, The Term negiah, Contemporary Formulations, Shaking Hands in Halacha