In The Librarian’s Book of Lists (Chicago: ALA, 2010), George Eberhart offers this definition:
"A library is a collection of resources in a variety of formats that is (1) organized by information professionals or other experts who (2) provide convenient physical, digital, bibliographic, or intellectual access and (3) offer targeted services and programs (4) with the mission of educating, informing, or entertaining a variety of audiences (5) and the goal of stimulating individual learning and advancing society as a whole." (p.1)
This definition is in turn compiled from:
(1) Heartsill Young, ed., The ALA Glossary of Library and Information Science (ALA, 1983)
(2) Robert S. Martin, "Libraries and Learners in the Twenty-First Century," Cora Paul Bomar Lecture, University of North Carolina at Greensboro, April 5, 2003.
(3) Deanna B. Marcum, "Research Questions for the Digital Era Library," Library Trends 51 (Spring 2003): 636-651.
Another general definition of library is from the Online Dictionary of Library and Information Science (ODLIS):
“Library -- from the Latin liber, meaning "book." In Greek and the Romance languages, the corresponding term is bibliotheca. A collection or group of collections of books and/or other print or nonprint materials organized and maintained for use (reading, consultation, study, research, etc.). Institutional libraries, organized to facilitate access by a specific clientele, are staffed by librarians and other personnel trained to provide services to meet user needs. By extension, the room, building, or facility that houses such a collection, usually but not necessarily built for that purpose. Directory information on libraries is available alphabetically by country in World Guide to Libraries, a serial published by K.G. Saur. Two comprehensive worldwide online directories of library homepages are Libdex and Libweb. See also the UNESCO Libraries Portal. Abbreviated lib. See also: academic library, government library, monastic library, new library, proto-library, public library, special library, and subscription library.
ANSI/NISO Z39.7-2013-Information Services and Use: Metrics & Statistics for Libraries and Information Providers : Data Dictionary : an American National Standard identifies categories for basic library statistical data reported at the national level, and provides associated definitions of terms” (p. 1), including the main types of libraries (public, academic, special, school, medical, etc.).
The American Library Association has also curated a listing of library related acronyms and initalisms.
Per the Institute for Museum and Library Services (IMLS), a public library is established under state enabling laws or regulations to serve a community, district, or region, and provides at least the following:
Public libraries continue to be places for education and self-help, and offer opportunity for people of all ages and backgrounds. They offer opportunity for everyone to learn and to pursue self-improvement. In response to community needs for information, many libraries offer such programs as English as a Second Language (ESL) classes, homework help, after-school programs for children, job information centers, assistance for new immigrants, literacy programs, and much, much more. To serve such community needs, public libraries collect and make available information in many, many formats.
Learning for life…whether the focus is on readiness for the next grade or college and career readiness, the school library program plays a crucial role in preparing students for informed living in the 21st century. The school library program provides learning opportunities that enable students to become efficient and effective in the pursuit of information.
Beyond its curricular role, the school library program gives each individual member of the learning community a venue for exploring questions that arise out of individual curiosity and personal interest. As part of the school library program, the school librarian provides leadership in the use of information technologies and instruction for both students and staff in how to use them constructively, ethically, and safely. The school librarian offers expertise in accessing and evaluating information, using information technologies, and collections of quality physical and virtual resources. In addition, the school librarian possesses dispositions that encourage broad and deep exploration of ideas as well as responsible use of information technologies. These attributes add value to the school community.
The school library represents for students one of our most cherished freedoms--the freedom to speak our minds and hear what others have to say. Students in America have the right to choose what they will read, view, or hear and are expected to develop the ability to think clearly, critically, and creatively about their choices, rather than allowing others to do this for them.
Academic libraries encompass research libraries, baccalaureate, masters and doctoral degree granting institutions, junior and community colleges, and distance learning programs of higher education. Academic libraries work together with other members of their institutional communities to participate in, support, and achieve the educational mission of their institutions by teaching the core competencies of information literacy—the abilities involved in identifying an information need, accessing needed information, evaluating, managing, and applying information, and understanding the legal, social, and ethical aspects of information use. The systematic delivery of instructional programs and services should be planned in concert with overall strategic library planning, including the library’s budgeting process. Such planning may also involve strategizing with other campus units to deliver collaboratively designed programming. Research has shown that the academic library is a positive influencing factor on students' academic success.