Hallux rigidus is a condition in which the big toe (‘hallux’) joint is stiff (‘rigidus’).
There are somewhat slightly different terminologies at play here. Some people use the term ‘hallux rigidus’ to mean that the joint is totally rigid and does not move. They use the term ‘structural hallux limitus‘ to refer to the joint that still has some movement but is less than the normal range. Others use the term ‘hallux rigidus’ to refer to a joint that is not necessarily rigid but has less range of motion than normal (rather than use the term ‘structural hallux limitus’ like others do).
The most common cause of hallux rigidus is osteoarthritis in the joint that could come from years of ‘wear and tear’ or could have come from a one-off trauma to the joint. Some people actually use the term ‘hallux rigidus’ to mean ‘osteoarthritis’.