This is the 4288th most frequent Russian word.
американскому
"American" (dative case, singular, masculine or neuter).
The word 'американскому' is in dative case, modifying 'послу', the noun it follows, to indicate the recipient of the action.
Я написал письмо американскому послу.
I wrote a letter to the American ambassador.
Here, 'американскому' describes 'учёному' in dative case, who is the indirect object of the verb.
Мы обсудили идею, предложенную американскому учёному.
We discussed the idea proposed to the American scientist.
In this usage, 'американскому' modifies 'проекту', connecting it to the action through the dative case for specific conventions.