New Words in Merriam-Webster Dictionary:

-- Americana (1841): genre of American music with roots in early folk and country music.

-- boomerang child (1988): young adult who returns to live at her or her family home, especially for financial reasons.

-- bromance (2004): a close nonsexual friendship between men.

-- continuous positive airway pressure (1975): abbreviated CPAP; a technique for relieving breathing problems (as those associated with sleep apnea or congestive heart failure) by pumping a steady flow of air through the nose to prevent the narrowing or collapse of air passages or to help the lungs to expand.

-- cougar (1774): slang term for a middle-aged woman seeking a romantic relationship with a younger man.

-- crowdsourcing (2006): the practice of obtaining needed services, ideas or content by soliciting contributions from a large group of people, especially from the online community, rather than from traditional employees or suppliers.

-- duathlon (1988): a three-part long-distance race typically having a running phase, a bicycling phase and a final running phase.

-- fist bump (1996): a gesture in which two people bump their fists together, as in greeting or celebration.

-- helicopter parent (1989): a parent who is overly involved in the life of his or her child.

-- m-commerce (1997): business transactions conducted by using a mobile electronic device, such as a cellphone.

-- parkour (2002): the sport of traversing environmental obstacles by running, climbing or leaping rapidly and efficiently.

-- robocall (1993): a telephone call from an automated source that delivers a pre-recorded message to a large number of people.

-- social media (2004): forms of electronic communication, such as websites for social networking and microblogging, through which users create online communities to share information, ideas, personal message and other content such as videos.

-- tweet (1768): a post made on the Twitter online message service.

-- walk-off (1990): ending a baseball game immediately by causing the winning run to score for the home team in the bottom of the last inning, i.e., a walk-off homer. Also, won by the home team in the bottom of the last inning, i.e., a walk-off win.
 

Words that are disappearing:

-- Alienism, the study and treatment of mental illness
 

-- Cyclogiro, a type of aircraft propelled by rotating blades

-- Charabanc, an early motor coach
 

-- Drysalter, a dealer in certain chemical products and foods

-- Supererogate, to do or perform more than is required
 

-- Succedaneum, something that is used as a substitute
 

-- Woolfell, the skin of a sheep or woolly-type animal with fleece attached
 

-- Wittol, a man who tolerates his wife's unfaithfulness