Wordnet 3.0
NOUN (4)
1. 
 person who walks in without having an appointment; 
- Example: "the emergency room was overrun with walk-ins"2. 
 an operative who initiates his own defection (usually to a hostile country) for political asylum; 
3. 
 an assured victory (especially in an election); 
[syn: walk-in, waltz]
4.  a small room large enough to admit entrance; 
ADJECTIVE (1)
1.  (of e.g. closets or refrigerators) extending very far enough back to allow a person to enter; 
- Example: "a deep walk-in refrigerator"
- Example: "walk-in closets"
WordNet (r) 3.0 (2006):
walk-in
    adj 1: (of e.g. closets or refrigerators) extending very far
           enough back to allow a person to enter; "a deep walk-in
           refrigerator"; "walk-in closets"
    n 1: person who walks in without having an appointment; "the
         emergency room was overrun with walk-ins"
    2: an operative who initiates his own defection (usually to a
       hostile country) for political asylum
    3: an assured victory (especially in an election) [syn: walk-
       in, waltz]
    4: a small room large enough to admit entrance