Yes, xi is a Scrabble word!
...and is worth 9 points in Scrabble, and 9 points in Words with Friends
xi
Adjective
1. Being one more than ten
Noun
1. The cardinal number that is the sum of ten and one
2. The 14th letter of the Greek alphabet
Adjective
1. Being one more than ten
Noun
1. The cardinal number that is the sum of ten and one
2. The 14th letter of the Greek alphabet