An implicit in Scala is a function applied or a parameter provided without explicitly appearing in the source code.
There are two variants of implicits in Scala: implicit conversions and implicit parameters.
Implicit conversions are applied by the scala compiler when it encounters an expression which it can not compile, but which can be compiled when one of the elements are passed to a function and the return value is used instead of the original element. A typical use case is the enrich-my-library pattern.
For example Scala allows the syntax "some.*regexp".r although the literal between the quotes is a String and String does not have a method r. But there is a method defined in Predef: augmentString(x: String): StringOps and StringOps has an r method. So this implicit conversion is applied.
Implicit parameters are parameters that are added to a method call by the compiler. A somewhat famous use case is in the collection library. Many methods accept a CanBuildFrom parameter which is used to define the type of collection returned by a method. In most cases this parameter is not specified explicitly but an implicit default value is used, which allows the collection library to return the "correct" specialized collection.
Only fields marked as such are considered for use as implicit parameters or conversions.
While this feature makes it possible to create powerful, concise and type safe DSLs and other APIs it should be used with care, since it also allows to create code that is rather hard to understand.
