What do \$V_{TN}\$ and \$V_{T}\$ stand for in a MOSFET?
Sir, in case of NMOS, we take threshold voltage Vtn ( it is positive) and in case of PMOS, we take threshold voltage Vtp (it is negative). Transistor is said to be OFF if:
NMOS OFF: VgsVtp 
In normal case, we always take NMOS as reference (its opposite is PMOS), so we can take Vtn as Vt. During calculations in linear and saturation region for PMOS, we can take |Vtp|.

These equations are in Book: CMOS DIGITAL INTEGRATED CIRCUITS (Analysis and Design) by
*SUNG-MO (STEVE) KANG
University of Illinois at Urbana- Champaign
*YUSUF LEBLEBIGI
Worcester Polytechnic Institute
Swiss Federal Institute of Technology-Lausanne