What are some basic words in English that are difficult to define?

Answer 1

Answer

Some words that are difficult to define are describe, visualization, solid, perpendicular.

I know that you probably know what all these words mean , mean it can be hard to define them without looking them up, therefore they are hard to define.

Answer

The most well known, difficult to define is the word up. It's used as an adverb, adjective, preposition, noun, verb, prefix and suffix. Dictionary.com lists 81 definitions, The Free Dictionary lists 65 definitions, and they each have a long list of idioms using up.