Viz.

I recently investigated the mysteries of "viz.," "sc.," "i.e.," and "e.g."  First, I'd advise against using any of these in your formal writing--they are shortcuts, and are best used as time-savers and space-savers when necessary in technical writing or very informal writing.  Use the spelled-out, plain English words I suggest below.  Future posts will cover "i.e." and "e.g."

"Viz." is an abbreviation of the Latin videlicet, which abbreviates videre licet--"one may read."  John Aubrey uses the word "viz." many times in this 1898 edition of Brief Lives (1696).  I'll use this book to illustrate the two situations in which "viz." is typically used:

1) to enumerate the members of a group just referred to:

"They consist of severall stately trees of the like groweth and heighth, viz. elme, chestnut, beach, hornbeame . . . " (pp. 79-80).

"When these verses were made she had three children by Sir George Kenelme, who are there mentioned, viz. Kenelme, George and John" (p. 231).

2) to reiterate more clearly something already said:

"His deepest divinity is where a man would least expect it: viz. in his Colloquies in a Dialogue between a Butcher and Fishmonger . . . " (p. 249).

This commonly used differentiation is based on the original Fowler's Modern English Usage.  "Viz." was used where you, as a modern writer, should use "namely," "that is to say," or even (if you're in a jovial mood) "to wit."  Another solution is not to use any word at all.  At the top of the last paragraph I might have written ". . . is typically used in two situations, viz. . . ."  but I omitted it.  I got a more succinct, better-flowing sentence.

Some usage notes if you take the plunge: Viz. isn't italicized, or capitalized; put a period after the z; put a comma before it; do not follow it with a comma.

"I.e.," and "e.g." are soon to come!

Fowler's Modern English Usage (1926) and R.W. Burchfield's The New Fowler's Modern English Usage (1996) are useful sources, as is the Wikipedia page.