Available since 1.2 (source)
  • (defprotocol name & opts+sigs)
A protocol is a named set of named methods and their signatures:
(defprotocol AProtocolName
   ;optional doc string
  "A doc string for AProtocol abstraction"
 :extend-via-metadata true
 ;method signatures
  (bar [this a b] "bar docs")
  (baz [this a] [this a b] [this a b c] "baz docs"))
 No implementations are provided. Docs can be specified for the
protocol overall and for each method. The above yields a set of
polymorphic functions and a protocol object. All are
namespace-qualified by the ns enclosing the definition The resulting
functions dispatch on the type of their first argument, which is
required and corresponds to the implicit target object ('this' in 
Java parlance). defprotocol is dynamic, has no special compile-time 
effect, and defines no new types or classes. Implementations of 
the protocol methods can be provided using extend.
 When :extend-via-metadata is true, values can extend protocols by
adding metadata where keys are fully-qualified protocol function
symbols and values are function implementations. Protocol
implementations are checked first for direct definitions (defrecord,
deftype, reify), then metadata definitions, then external
extensions (extend, extend-type, extend-protocol)
 defprotocol will automatically generate a corresponding interface,
with the same name as the protocol, i.e. given a protocol:
my.ns/Protocol, an interface: my.ns.Protocol. The interface will
have methods corresponding to the protocol functions, and the
protocol will automatically work with instances of the interface.
 Note that you should not use this interface with deftype or
reify, as they support the protocol directly:
 (defprotocol P 
  (foo [this]) 
  (bar-me [this] [this y]))
 (deftype Foo [a b c] 
  (foo [this] a)
  (bar-me [this] b)
  (bar-me [this y] (+ c y)))

(bar-me (Foo. 1 2 3) 42)
=> 45
  (let [x 42]
    (reify P 
      (foo [this] 17)
      (bar-me [this] x)
      (bar-me [this y] x))))
=> 17