Matches documents that have fields containing terms with a specified prefix (not analyzed). The prefix query maps to Lucene PrefixQuery. The following matches documents where the user field contains a term that starts with ki:

GET /_search
{ "query": {
    "prefix" : { "user" : "ki" }
  }
}

A boost can also be associated with the query:

GET /_search
{ "query": {
    "prefix" : { "user" :  { "value" : "ki", "boost" : 2.0 } }
  }
}

This multi term query allows you to control how it gets rewritten using the rewrite parameter.