In normal terminology we say that both the parse and derivation trees are same in meaning so if a grammar derives one string with left derivation as well as right derivation then it is ambiguous , if both left and right derivation correspond to 2 different parse trees.
Now what if I have the left-most derivation tree to be similar to right-most derivation tree ,and they both correspond to 1 parse tree so can I conclude that the grammar is unambiguous ?
An unambiguous grammar has same leftmost and rightmost derivation:
Ambiguous Grammars
Definitions:
If a grammar has more than one leftmost derivation for a single sentential form, the grammar is $\color{Blue}{\text{ambiguous}.}$
If a grammar has more than one rightmost derivation for a single sentential form, the grammar is $\color{Blue}{\text{ambiguous}.}$
The leftmost and rightmost derivations for a sentential form $\color{Red}{\text{may differ,}}$ even in an $\color{Green}{\text{unambiguous}}$ grammar.
Ref@https://www.eecis.udel.edu/~cavazos/cisc471-672/lectures/Lecture-08.pdf
Comment : Here, last point says an unambiguous grammar has same (or may not be) leftmost and rightmost derivation.
On the other hand, if a grammar has more than one parse tree for any string in given language, then grammar said to be ambiguous.