From Quake Wiki
entity find (entity start, .string field, string match)
start = begining of list to search (world, for the begining of list)
field = entity field that must be examined (ex: targetname)
match = value that must be matched (ex: other.target)
returns the entity found, or world if no entity was found.
Searches the server entity list beginning at start, looking for an entity that has entity.field = match
Example: find the first player entity:
e = find( world, classname, "player");
Take care that field is a name of an entity field, without dot, and without quotes.
In the example above find(...) was used because find works for entity fields that are of type string . And the field that was looked up (classname) was defined somewhere as: .string classname