A comprehensive reference work covering all branches of knowledge or a specific field, typically arranged alphabetically or thematically.
I bought an encyclopedia for children.
This encyclopedia consists of 30 volumes.
If there's a word you don't understand, look it up in an encyclopedia.
That animal is covered in detail in the encyclopedia.
辞書 is a general dictionary you consult for word meanings and spellings, while 百科事典 is an encyclopedia you consult for detailed knowledge on topics.
百科事典 is typically counted with 冊 when referring to individual volumes of the set.
百科事典 is counted with 巻 when referring to a specific numbered volume within the full set.