PKA may refer to:

• Professionally Known As (referring to pen names, stage personas and similar)
• pK_a, the symbol for the acid dissociation constant at logarithmic scale
• Protein kinase A, a class of cAMP-dependent enzymes
• Pi Kappa Alpha, the North-American social fraternity
• Public key authentication, establishing key authenticity in public-key cryptography 
• Professional Karate Association
• Primary knock-on atom
• Russian Federal Space Agency 

An acid dissociation constant, K_a, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions. In aqueous solution, the equilibrium of acid dissociation can be written symbolically as:

where HA is a generic acid that dissociates into A^-, known as the conjugate base of the acid and a hydrogen ion which combines with a water molecule to make an hydronium ion. In the example shown in the figure, HA represents acetic acid, and A^- represents the acetate ion, the conjugate base.

Pi Kappa Alpha (ΠΚΑ) or Pike is a college fraternity founded at the University of Virginia in 1868. The fraternity has over 220 chapters and colonies across the United States and Canada with over 15,500 undergraduate members and 270,000 lifetime initiates. Originally a white-only southern organization, the fraternity became a national fraternity in 1909, and removed its racial restrictions in 1964.