plug-in
1 of 3adjective
ˈpləg-ˌin 
: designed to be connected to an electric circuit by plugging in
a plug-in toy
a plug-in circuit board
plug-in
2 of 3noun
1
: something that plugs in
2
: a small piece of software that supplements a larger program (such as a browser)
plug in
3 of 3verb
plugged in; plugging in; plugs in
intransitive verb
: to establish an electric circuit by inserting a plug
transitive verb
: to attach or connect to an electric receptacle (such as an outlet)
Word History
First Known Use
Adjective
1922, in the meaning defined above
Noun
1946, in the meaning defined at sense 1
Verb
1903, in the meaning defined at intransitive sense