The trustless definition can be taken as a system like a blockchain where none of the participants have any authority (there is no central power), but the consensus is achieved between them without trusting each other.
Understanding the term
The concept of trustlessness later became the preface of cryptocurrencies. In a trustless system, two parties that do not trust each other can conveniently reach a consensus in a secure environment where no central authority is active.
Being the first-ever cryptocurrency, Bitcoin was created as a flagship digital asset whose foundation was based on a trustless system. What it means is that all transaction-related data can be verified and stored on the blockchain, which is a distributed ledger in the case of Bitcoin and other cryptocurrencies.
From the trustless meaning, it can be easily gathered that such systems operate differently from traditional banks or other centralized approaches found in our society. Trustless systems offer superior security against manipulation and alteration of data, which is commonly observed in a system where third parties are involved in making decisions. Still, centralized systems are more popular than trustless ones.
Any environment that is free of centralized authority can be called trustless. But this concept truly began with Bitcoin, the first ever peer-to-peer digital currency.