American Sign Language, also known as ASL or Ameslan (old term), is a language in visual-spatial modality. It is dominantly used by culturally Deaf people in the U.S. and speaking-English Canada. ASL is the 3rd (then 4th) most studied language in the U.S., according to the Modern Language Association's statistics (2016).
The Yale alumnus and minister Thomas Hopkins Gallaudet (1787-1851) met a neighbor Dr. Cogswell's little deaf daughter, Alice Cogswell, in his neighborhood in Hartford, Connecticut. The legend is that Gallaudet wrote a word "hat" in the soil and pointed to the hat. Alice learned the word. This inspired the legacy of deaf education in America.
There were no deaf schools in the U.S. at that time. Alice's father Dr. Cogswell and Gallaudet were interested in founding deaf education. With the help of funds, Gallaudet agreed to travel to to England to learn about teaching methods of deaf education. He visited the Braidswood family-owned private oral-method school. Unsatisfied; however, a fate twisted that led him to a lecture by French educators (the school head Abbe Sicard and the deaf faculty members Laurent Clerc and Jean Massieu) from Paris.
There Gallaudet was invited to visit the school for the deaf -- Institut National de Jeunes Sourds de Paris (the National Institute for Deaf Children in Paris) founded in 1760 which is the first public school for the deaf in the world. He spent some time at the school to learn the teaching methods. As the funds ran out, he needed to go back to America.
Gallaudet asked the brilliant young deaf teacher, Clerc, to come to America with him. Back in America, they founded the first permanent school for the deaf, American Asylum (now the American School for the Deaf) in Hartford, Connecticut in 1817. Over the decades, schools for the deaf popped up all over North America.
The Old French Sign Language (OFSL) that Clerc brought to the U.S. was intermingled with the native signed languages, including the Martha Vineyard Sign Language. ASL has been evolved ever since then.
Deaf education and sign language prospered as well as professions that deaf people of all walks enjoyed until the "Dark Age" from the infamous Milan of 1880 where the oralist delegates voted to ban sign language used in deaf education around the world until the 1960s. The successes and professions of deaf lives declined. Deaf people and their language suffered in face of the systemic oralist oppressions.
Despite the oralist oppression, ASL thrived on in Deaf community and sign language was still used outside classrooms where Deaf children's tiny hands were commonly slapped with a ruler for using sign language. Like anywhere in the world, there was much ignorance. Signed languages were seen and treated as bad with a condescending attitude. Deaf people and their signed languages survived through the systemic oppressions. Slowly, various English signed systems were introduced in deaf education to "support" English speech through visual methods. Again, it doesn't support the natural, native language acquisition.
In the 1960s, William Stokoe, a hearing English professor at Gallaudet University (formerly Gallaudet College), observed ASL sign language used by the Gallaudet students on campus. He studied and discovered that it had linguistic features (phonology, morphology, syntax, and all) like any spoken language. He did his research work. He proclaimed that it is a natural language. His works were published in linguistics journals. Since the 1970s, a number of research and studies have quickly grown. Stokoe is known as the father of ASL linguistics. This was a life-changing moment for Deaf people who reclaimed their pride in language and culture.
In the 1980s, credit courses in ASL bloomed in colleges and universities as well as in high schools across North America. In the meanwhile, bilingual-bicultural education (ASL and English) was introduced into deaf schools in the 1980s. The bilingual approach has been all successful.
In the 1990s and onward, cognitive neuroscientist Dr. Laura-Ann Petitto (1954-) is known for her scientific discoveries involving early language acquisition in sign language, pronoun acquisition in language development, manual babbling, and others. Her studies show that language is amodal, which means that language is independent from modality -- the brain doesn't tell the difference between hands and lips. This was a revolutionary on the nature of language.
Today millions of Deaf people, friends, colleagues, and family members speak ASL on a daily basis. ASL has been popular for a second language learning in post-secondary education. There are many ASL courses provided in secondary and post-secondary education as well as continuing education programs in North America.
The public (hearing) attitude toward sign language and Deaf people have changed with more allyship and embrace for diversity and equality/equity. However, there is still a practice of audist attitude and systemic oppressions (linguicism, audism, phonocentrism, and likes), especially in the medical niches. We still fight against other -isms, including cultural appropriation.
Deaf people of all walks are proud bilingual-bicultural (or even multilingual-cultural). What unite Deaf people with intersectional identities all together with the common thread is Deafhood and the (signed) language -- the core identity of Deaf people.
"The Legacy Begins." https://www.gallaudet.edu/about/history-and-traditions/the-legacy-begins/