What is the difference between a compound word and a portmanteau?

Compound words and portmanteaus are both formed by joining multiple words, but there are a few differences between the two:

• Compound words retain all the letters from both original words and don’t always express meanings from both words (e.g., butter + fly = butterfly). A compound word is treated as a distinct word with its own dictionary entry.
• Portmanteaus omit letters from the original words (e.g., Spanish + English = Spanglish) to combine the original words’ sounds and meanings. Portmanteaus are often treated as casual or informal, though some become widely accepted and have their own dictionary entries.