Meaning of -ER in English

-ER

suffix

1.

(in nouns) a person or thing that :

lover

computer

—compare -ee , -or

2.

(in nouns) a person or thing that has the thing or quality mentioned :

three-wheeler

foreigner

3.

(in nouns) a person concerned with :

astronomer

philosopher

4.

(in nouns) a person belonging to :

New Yorker

5.

(makes comparative adjectives and adverbs):

wider

bigger

happier

sooner

—compare -est

Oxford Advanced Learner's English Dictionary.      Оксфордский английский словарь для изучающик язык на продвинутом уровне.