A property list (or plist) is another way of representing a mapping from keys to values. Instead of the list consisting of conses of a key and a value, the keys and values alternate as successive entries in the list. Thus, the association list
((a . 1) (b . 2) (c . 3))
has the equivalent property list form
(a 1 b 2 c 3)
Property lists are used to represent the properties associated with various sorts of objects, such as symbols, strings, frames, etc. The convention is that property lists can be modified in-place, while association lists generally are not.
Plists come in two varieties: normal plists, whose keys are
compared with eq, and lax plists, whose keys are compared
with equal,