{# defined works with variable names #}
{% if foo is defined %}
...
{% endif %}
{# and attributes on variables names #}
{% if foo.bar is defined %}
...
{% endif %}
{% if foo['bar'] is defined %}
...
{% endif %}
When using the defined test on an expression that uses variables in some method calls, be sure that they are all defined first:
{% if var is defined and foo.method(var) is defined %}
...
{% endif %}