TITLE: "nice functions"
QUESTION [7 upvotes]: I see the statement of "nice functions" in textbooks and the authors usually don't need to give the definition of "nice functions". For example in a book which I read now the authors write
"Morrey spaces is not separable. A version of Morrey space where it is possible to approximate by "nice functions" is vanishing Morrey space." 
and don't give the definition of "nice functions" anywhere in the book.
I wonder in here what is the meaning of "nice functions" ? 
and
Is there a fixed definition of "nice functions" ?

REPLY [7 votes]: Comment converted to answer:
I read "nice" or "well-behaved" as "satisfying all requirements for the appropriate theorems I am using."