How do you make Twitter Bootstrap Accordion keep one group open?

Here is an easy way to do it:

Bootstrap 4

$('.accordion .btn-link').on('click', function(e) { 
  if (!$(this).hasClass('collapsed')) { 
    e.stopPropagation(); 
  } 
});

from @mr_joncollette in the comments

Bootstrap 3

JsFiddle for Bootstrap 3.

Code for Bootstrap 3:

$('.panel-heading a').on('click',function(e){
    if($(this).parents('.panel').children('.panel-collapse').hasClass('in')){
        e.stopPropagation();
    }
    // You can also add preventDefault to remove the anchor behavior that makes
    // the page jump
    // e.preventDefault();
});

The code checks if the clicked element is the one that is currently shown (by the class "in") and if it does have the "in" class, it stops the hiding process.

Deprecated Bootstrap 2

JsFiddle for Bootstrap 2.

Code for Bootstrap 2:

$('.accordion-toggle').on('click',function(e){
    if($(this).parents('.accordion-group').children('.accordion-body').hasClass('in')){
        e.stopPropagation();
    }
    // You can also add preventDefault to remove the anchor behavior that makes
    // the page jump
    // e.preventDefault();
});

Note: Be careful if you want to attach more click events on the accordion, since the e.stopPropagation() will block events that would occur after the check.

I want to precise @Hugo Dozois 's answer

http://jsfiddle.net/SMT9D/61/

You should add e.preventDefault(); to prevent the default behaviour of # HTML anchor if you have a scroll in your page

$('.panel-heading a').on('click',function(e){
    if($(this).parents('.panel').children('.panel-collapse').hasClass('in')){
        e.preventDefault();
        e.stopPropagation();
    }
});

Updated 2018

Here's how to keep at least open in both Bootstrap v3 or v4. This means that the open accordion can only be closed by toggling another one open.

Bootstrap 4

https://www.codeply.com/go/bbCcnl0jBB

// the current open accordion will not be able to close itself
$('[data-toggle="collapse"]').on('click',function(e){
    if ( $(this).parents('.accordion').find('.collapse.show') ){
        var idx = $(this).index('[data-toggle="collapse"]');
        if (idx == $('.collapse.show').index('.collapse')) {
            e.stopPropagation();
        }
    }
});

Also, see this answer which shows how to specify a "default" accordion that will open when all others are closed.

Bootstrap 3

$('[data-toggle="collapse"]').on('click',function(e){
    if($(this).parents('.panel').find('.collapse').hasClass('in')){
        var idx = $(this).index('[data-toggle="collapse"]');
        var idxShown = $('.collapse.in').index('.accordion-body');
        if (idx==idxShown) {
            e.stopPropagation();
        }
    }
});

https://www.codeply.com/go/yLw944BrgA

<div class="accordion" id="myAccordion">
    <div class="panel">
        <button type="button" class="btn btn-danger" data-toggle="collapse" data-target="#collapsible-1" data-parent="#myAccordion">Question 1?</button>
        <div id="collapsible-1" class="collapse">
            ..
        </div>
    </div>
    <div class="panel">
        <button type="button" class="btn btn-danger" data-toggle="collapse" data-target="#collapsible-2" data-parent="#myAccordion">Question 2?</button>
        <div id="collapsible-2" class="collapse">
            ..
        </div>
    </div>
    <div class="panel">
        <button type="button" class="btn btn-danger" data-toggle="collapse" data-target="#collapsible-3" data-parent="#myAccordion">Question 3?</button>
        <div id="collapsible-3" class="collapse">
           ...
        </div>
    </div>
</div>

(Note: the panel class is needed in Bootstrap 3 to make the accordion behavior work)