Loose Brethren is a disparaging term sometimes used to identify the Open Brethren assemblies within the Plymouth Brethren movement which hold to a more open policy of reception to Communion (Bible Chapel Brethren), in contrast to the more conservative and often more traditional Gospel Hall Brethren (sometimes called Tight Brethren).
These assemblies are also sometimes referred to as "Wide Opens", although there are varying degrees of policy with regard to reception to Communion within the "Bible Chapel Brethren". Reception is generally on the basis of the biblical understanding of the "one body of Christ", but it ranges in practice from those that will receive believers from Bible Chapels and Gospel Halls with a letter of commendation only, to those that see the Lord's Table for Communion as "completely open." This is in contrast to the teaching of the Gospel Hall Open Brethren that reception to the Lord's Table for Communion is based on assembly fellowship.
The term Loose Brethren also has a wider application encompassing a conservative approach to church life and a greater regulation of church order than those assemblies deemed 'tight'. For example their (historical) use of musical instruments, singers, interdenominational preaching and (nowadays) to their greater toleration of innovation: women not wearing head-coverings, resident full-time workers (or pastors), family services, praise bands, PowerPoint, female participation etc. etc. All of which, of course, are cause of criticism from the more traditional.
In Peter Cousins's 'The Brethren' (1982) [1] he made up the fictitious South Street Gospel Hall and Westbrook Chapel, describing them as 'tight' and 'loose' assemblies. The account illustrates the diversity of the Open Brethren.
The pejorative term 'Loose Brethren' would never be used as a self-descriptor by a Brethren member - more positive terms such as 'open' or 'ecumenical' or 'independent evangelical' would be used.