Atomic swap
The problem of atomic cross-chain trading is one where (at least) two parties, Alice and Bob, own coins in separate cryptocurrencies (e.g. Bitcoin and Litecoin), and want to exchange them without having to trust a third party (centralized exchange).
A non-atomic trivial solution would have Alice send her Bitcoins to Bob, and then have Bob send Litecoins to Alice - but Bob has the option of going back on his end of the bargain and simply not following through with the protocol, ending up with both Bitcoins and Litecoins.
This problem can be solved using Contracts and nLockTime.
