Word for the thief's key that can unlock anything
Skeleton key has been suggested by many other answers, but always as a secondary suggestion. That would be incorrect: it is the correct answer here. No other answer suggested, possibly excepting the archaic passe-partout, has this meaning.
A skeleton key is a key which has been specially made so that it will open many different locks.
(Collins)
Many dictionaries, as well as Wikipedia, will tell you about the original design of a skeleton key: a key with all the ridges filed away so that it can open many locks. It’s actually where the name comes from, as stripping off the ridges was said to leave only the key’s “skeleton.” Anyway, that design only works on rather basic locks known as warded locks. Warded locks are not much in use anymore, largely because of the vulnerability to skeleton keys, therefore the original skeleton key design is not generally all that useful for theft.
On the other hand, the term skeleton key has outlived the original design, and is often used to describe a hypothetical key that opens any lock, rather than just those susceptible to the original skeleton design. For instance,
Many times refers to a magical key that will sucessfully unlock anything.
(Urban Dictionary—note that by the third definition, the term has become a euphemism for sex, as most Urban Dictionary definitions eventually end up doing)
Used metaphorically, then, skeleton key means a key that will open any lock. Given the sophistication of modern locks, and the many different designs for them, this is generally treated as a magical ability, since no actual key could open all of those different locks. In my experience, the metaphorical usage of skeleton key is far more common than the specific historical design that it is named after.
Master Key
A master key is a key which will open all the locks in a set, even though each lock has its own different key.
(Collins Dictionary)
Master keys are legitimate tools intentionally used by property owners, not thieves’ tools. For example, if you own a hotel, you might have the locks for your doors made with a master key, and then give it to the cleaning staff.
Generally speaking, the master key has to be made first—if you just have a random set of locks it will be extremely difficult, and probably just impossible, to come up with a master key for all of them.
If a thief got their hands on a master key for a building or whatever that they wished to rob, it makes the theft very easy, of course, but ultimately this is no different from stealing the key or keys that open whatever you need. The master key is easier because it’s one key that opens many locks.
Pass Key
The word pass key can mean master key, or it can mean skeleton key. When used to mean skeleton key, it can have this meaning, but the fact that it can instead mean master key makes it less than useful here.
- MASTER KEY
- SKELETON KEY
(Meriam-Webster)
Universal Key
Universal key is not a common term, and I can find no dictionary that includes it. A quick Google search suggests that by far the most common usage for this term is a “universal handcuff key,” a simple cylinder with a little “flag.” Per comments by Aziris Morora and mrog, handcuffs all use this same, simple design, so you can actually get a standardized key that will open all of them. But it won’t open anything else.
Anyway, not what you are looking for.
Lock picks and bump keys
On the other hand, lock picks and bump keys are definitely used as theives’ tools, as well as used legitimately by locksmiths. They can be used to open up locks with some skill and effort. Many locks are very easy to pick; many locks are nearly, if not outright, impossible to pick. Either way, lock picking is a skill and these tools help; they don’t just open the lock automatically for you.
Importantly, even though a bump key is called a “key,” it doesn’t really work like one—you can’t just stick it in, turn it, and expect the lock to open. Instead, you have to “bump” the lock to get it to open. However, it’s worth noting that the bump key does bear some similarities to the original design of the skeleton key—like the skeleton key was filed smooth to foil cylinder locks, the bump key consists of several unusually-sharp points which can be used to “bump” pin–tumbler locks. Like the original skeleton key, the bump key is not useful for other lock designs. Unlike skeleton key, though, bump key has not reached the quasi-legendary status that skeleton key has, and it is not used for a hypothetical key-for-all-locks.
The things that thieves actually use to open doors is called a lockpick or picklock. These are actually quite tricky to use, and won't work on all locks, although they do work on many of them.
A master key is something that is made to open all the locks in one building or set of buildings. Generally, managers and janitors have these. But these (or at least, the ones in actual use) don't open all locks; just locks that have been made so as to be opened by this specific key.
A skeleton key is a key that has been (probably illicitly) filed down so that it will open all of some particular class of locks. I don't believe that skeleton keys work on most modern locks, but they do work on old-fashioned types of locks, and so in the past they would have been very useful to thieves.
If you are talking about some mythical key that opens all doors easily, I don't believe there is such a myth in the English-speaking world. If such a myth actually existed, it might be called a
universal key,
and in fact, Googling universal key shows that it indeed has been used with this meaning
You are probably looking for passe-partout:
something that secures entry everywhere, esp a master key
Also skeleton key or passkey:
A skeleton key is a key which has been specially made so that it will open many different locks.
(Collins Dictionary)