define-library

(define-library (name...) declaration...)

Program syntax, exported by no library: defines a library that a program, or another library, imports by its name. A name is a list of identifiers and exact non-negative integers; names beginning with scheme are reserved. The declarations are:

(export spec...), a spec being an identifier or (rename internal external);
(import import-set...), what the library's body sees -- everything, as in a program with no import, when the library imports nothing;
(begin body...);
(include "file"...) and (include-ci "file"...), which add a file's contents to the body;
(include-library-declarations "file"...), whose file holds more declarations;
(cond-expand (requirement declaration...)...), the declarations of the clause cond-expand takes.

The body is lowered like a file of its own. Only the exported names reach an importer; every other top-level name stays private to the library, so a program may define the same name. A library is found among the define-library forms at the beginning of the importing file, before its imports, or as a file: (import (geometry point)) reads geometry/point.sld, else geometry/point.scm, in the directory of the file the program started from. It runs once, when the first file imports it, however many files import it. A library may export a define-syntax macro: a name its template uses freely means what it meant in the library, a private one included, whatever the importer calls that name.

A library in its own file, with a private helper and an export under another name:

; file: geometry/point.sld
(define-library (geometry point)
  (export make-point point-x point-y (rename add point-add))
  (import (scheme base))
  (begin
    (define-record-type point (make-point x y) point? (x point-x) (y point-y))
    (define (add a b)
      (make-point (+ (point-x a) (point-x b)) (+ (point-y a) (point-y b))))))

(11 22 mine)

A macro exported with the library's private procedure and variable, which the importer's own count! does not replace:

; file: stack/macros.sld
(define-library (stack macros)
  (export push! pushes)
  (import (scheme base))
  (begin
    (define pushes 0)
    (define (count!) (set! pushes (+ pushes 1)))
    (define-syntax push!
      (syntax-rules ()
        ((_ x place) (begin (count!) (set! place (cons x place))))))))

((2 1) 2 mine)