"A named collection of one or more database files that forms a single unit of allocation and administration. Filegroups enable the creation of objects in a specific place - for example, placing a heavily accessed table on a very fast drive. They also provide the ability to back up specific objects." (Microsoft Corporation, "SQL Server 7.0 System Administration Training Kit", 1999)
"A named collection of one or more files that forms a single unit of allocation or for administration of a database. Filegroups allows you to engage in advanced administration techniques including the specific storage of particular objects on particular hard drives." (Anthony Sequeira & Brian Alderman, "The SQL Server 2000 Book", 2003)
"A named collection of one or more disk files that represent a single allocation on or for administration of a database. A database can have more than one filegroup." (Allan Hirt et al, "Microsoft SQL Server 2000 High Availability", 2004)
"In SQL Server, a named collection of one or more files that forms a single unit of allocation. Also for administration of a database." (Thomas Moore, "EXAM CRAM™ 2: Designing and Implementing Databases with SQL Server 2000 Enterprise Edition", 2005)
"This is a logical grouping of files used to segregate database objects for storage and performance reasons." (Joseph L Jorden & Dandy Weyn, "MCTS Microsoft SQL Server 2005: Implementation and Maintenance Study Guide - Exam 70-431", 2006)
"A logical means of controlling the placement of database objects on a file or set of files." (Marilyn Miller-White et al, "MCITP Administrator: Microsoft® SQL Server 2005 Optimization and Maintenance 70-444", 2007)
"A method of optimizing the performance of a database by controlling the placement of database files and database objects. By default, all data and database objects are placed into a single file, in a single filegroup. Unlike files, which can be viewed on the disk, a filegroup is conceptual." (Darril Gibson, "MCITP SQL Server 2005 Database Developer All-in-One Exam Guide", 2008)
"Collections of SQL Server data files. For performance and administrative reasons, you can place user objects into dedicated filegroups." (Robert D Schneider & Darril Gibson, "Microsoft SQL Server 2008 All-in-One Desk Reference For Dummies", 2008)
"A named collection of one or more data files that forms a single unit of data allocation or for administration of a database." (Microsoft, "SQL Server 2012 Glossary", 2012)
