Answer:

SIGMA

Answer	Crossword Clue
SIGMA	Greek letter
SIGMA	Standard deviation symbol
SIGMA	Symbol of summation

SIGMA	Greek consonant
SIGMA	Campus letter
SIGMA	Frat letter
SIGMA	Rho follower
SIGMA	Tau preceder
SIGMA	Letter after rho
SIGMA	S, as in Socrates
SIGMA	Greek letter meaning "sum" in math

SIGMA	"The Sweetheart of ... Chi"
SIGMA	Letter on a frat house
SIGMA	Start for Socrates?
SIGMA	Sum symbol
SIGMA	Calculus symbol
SIGMA	Rho-tau connection
SIGMA	[XXX]
SIGMA	Part of SASE

SIGMA	18th Greek letter
SIGMA	Summation symbol in math
SIGMA	Letter between rho and tau
SIGMA	Frat house letter
SIGMA	Summation symbol, in math
SIGMA	Sorority letter
SIGMA	Sum symbol, in math
SIGMA	It's halfway between pi and upsilon

SIGMA	Sum symbol in math
SIGMA	Summation symbol
SIGMA	Either "E" in "Greek" in the poster for "My Big Fat Greek Wedding"
SIGMA	Fraternity letter
SIGMA	Letter on the AutoSum button in Excel
SIGMA	Greek letter.
SIGMA	Greek "S."
SIGMA	... Xi, fraternity for science research.

SIGMA	... Chi.
SIGMA	Wally's ... 7.
SIGMA	... VII.
SIGMA	Fraternity letter.
SIGMA	Millisecond