lib/less/parser/parser.js
function () {
                var path, args, options,
                    index = parserInput.i,
                    dir   = parserInput.$re(/^@plugin?\s+/);

                if (dir) {
                    args = this.pluginArgs();

                    if (args) {
                        options = {
                            pluginArgs: args,
                            isPlugin: true
                        };
                    }
                    else {
                        options = { isPlugin: true };
                    }

                    if ((path = this.entities.quoted() || this.entities.url())) {

                        if (!parserInput.$char(';')) {
                            parserInput.i = index;
                            error('missing semi-colon on @plugin');
                        }
                        return new(tree.Import)(path, null, options, index, fileInfo);
                    }
                    else {
                        parserInput.i = index;
                        error('malformed @plugin statement');
                    }
                }
            }
function () {
                var path, args, options,
                    index = parserInput.i,
                    dir   = parserInput.$re(/^@plugin?\s+/);

                if (dir) {
                    args = this.pluginArgs();

                    if (args) {
                        options = {
                            pluginArgs: args,
                            isPlugin: true
                        };
                    }
                    else {
                        options = { isPlugin: true };
                    }

                    if ((path = this.entities.quoted() || this.entities.url())) {

                        if (!parserInput.$char(';')) {
                            parserInput.i = index;
                            error('missing semi-colon on @plugin');
                        }
                        return new(tree.Import)(path, null, options, index, fileInfo);
                    }
                    else {
                        parserInput.i = index;
                        error('malformed @plugin statement');
                    }
                }
            }
A @plugin directive, used to import plugins dynamically.  @plugin (args) "lib";