The Greek capital letter for Xi is Ξ.
The Greek capital letter for Xi is Ξ.
Xi is the fourteenth letter of the Greek alphabet.
The Greek capital letter for Delta is Δ.
The Greek capital letter for Omega is Ω.
The Greek capital letter for Phi is Φ.
The Greek capital letter for Theta is Θ.
The Greek capital letter for Psi is Ψ.
The Greek capital letter for Sigma is Σ.
The Greek capital letter for Gamma is Γ.
The Greek capital letter for Lamda is Λ.
The Greek capital letter for Pi is Π.
YES, XI is acceptable (it's a Greek letter). [HINT: All of the letters of the Greek alphabet are acceptable words in Scrabble.]
It is a noun, the 14th letter of the Greek alphabet.