At his best friend’s wedding reception on the California coast, David J. Peterson stood to deliver his toast as best man. He held his Champagne glass high and shouted “Hajas!” The 50 guests raised their glasses and chanted “Hajas!” in unison.

The word, which means “be strong” and is pronounced “hah-DZHAS,” has great significance for Mr. Peterson. He invented it, along with 3,250 other words (and counting), in the language he created for the HBO fantasy series “Game of Thrones,” called Dothraki.

Some people build model railroads or re-enact Civil War battles; Mr. Peterson, a 30-year-old who studied linguistics at the University of California, San Diego, is a “conlanger,” a person who constructs new languages. Until recently, this mostly quixotic linguistic pursuit, born out of a passion for words and grammatical structures, lived on little-visited Web sites or in college dissertations.

Today, a desire in Hollywood to infuse fantasy and science-fiction movies, television series and video games with a sense of believability is driving demand for constructed languages, complete with grammatical rules, a written alphabet (hieroglyphics are acceptable) and enough vocabulary for basic conversations.