I don't understand how this is a Book Move.

No there is no confusion. The engine refer to it's library as a book move. It is the engine we are talking about. As far as I know we do not have access to the book the chess.com engine uses for openings. What other people refer to as a book move is more obscure: Anything from any chess book could be considered a book move. That basically means that what was considered a book move yesterday might be not a book move today. ECO is GM practice commented by GM's.

 But what most people refer to when they use " book move " are moves from trusted/reputable opening books such as the set of ECOs and the various chess "Bibles" like : MCO, NCO and BCO.

I think using databases for our opening moves may be fraught with danger and unreliable moves. See this related thread : http://www.chess.com/forum/view/general/data-base-moves-versus-book-moves

Yes, I know and it is a very careless way of using the word book. If you take ECO for instance any comment or game is made or played by someone. So a more correct way of saying book move is to refer to the player or commentator. That is what makes a "book move" valuable is if a GM or expert states something specific about a move. The move has to be "certified" so to speak. If you can not name tag a move it isn't a book move.

Polishing a raw database into a tight opening "book" is practically (actually?) it's own separate chess hobby!  Big sloppy databases like chess.com's game explorer are perfectly fine for getting ideas and simple research, but absolutely it and other even bigger sloppier databases are fraught with danger and unreliable moves.

"Book" generally (to me at least) means "certified and approved by the authorities" ... but it sometimes it just means "studied" or "well-known"... or...shows up in our db a lot.

Moves one through 4 are indeed a known line of th sicilian.

Afaf: you said: I couldn't believe that my opponent's moves 4 and 5 were being called "book moves" by the computer despite resulting in the loss of a pawn.

Move 5 looks like a blunder -- losing the pawn without compensation... but move 4 is not a blunder, doesn't lose the pawn, and is in fact book. There is a tactic that returns the pawn, with interest. You should search the position after 4...Nxe4?? until you find the tactic. It is a very common way to lose a game. Whenever you seem to be gifted a pawn in the opening -- look for this tactic, or similar, before biting on the pawn.

Chess.com engine refers the opening lines with a ECO number as a book move. You can get the Encyclopaedia of Chess Openings, and find all those openings there. 

No reputable engine will name a random opening move as a book move just because that move was played by somebody in a random game. Known openings are already in ECO, and that is whats taken as a reference. 

As a conclusion, if an opening line is called as "book move" it needs to have an ECO number, an opening name, and in most cases a variation name. 

In Afaf's example it's ECO number B50, opening name Sicilian Defense, Delayed Alapin Variation. 

Hey, sorry about that. Ply is a half move?  I thought ply meant 2 moves...the white and black move.  Didn't mean to be vague. 

MM78 correctly interpreted my meaning. 

The Computer Analysis listed 5. 0-0 as Book Move. 

I was wondering about whether there is some tactic after move 4...and I think Reb and someone else have given the explanation for why the pawn can't be taken after move 4.  I'll have a look at that now.

**Honestly...the database on this site is so bonkers.  Does anyone know of a reliable database site or method?  Please message it to me privately so it's not an affront to the site staff.

 The tactic nets a piece for the pawn and is easy to see... 5 Qa4+ .... returns the pawn with interest ?

Hermes3,

Oh I have most fide ECO's since the end of the 70's but the book version is outdated now. ECO is just a classification system just as informator before that. In order to be a book move you need an author with some reput.

@Reb... "returns the pawn with interest" is an incorrect phrase... I meant: You get better than a pawn back. Yes, reb, yes, I'm really not surprised a bit that the Qa5+ tactic is easy for an NM to spot -- golly -- that's why I directed my comment @Afaf, who didn't see it -- I was rather hoping she'd look for it. But at least you got in some practice pointing out the obvious.

 Apparently you didnt read the entire thread as it was already pointed out before you made your post challenging her to find the tactic.

It is me who misunderstood it then.

Not only that 5. 0-0 is not a book move, there is also no games played with fifth move castling in chess.com database. You can see the moves played after 4.Nc6 below. If that is the case staff needs to take a look at the glitch.

move	games	white wins / draw / black wins
5.d4	251	59% 21.5% 19.5%
5.d3	95	43.2% 29.5% 27.4%
5.Qc2	10	50% 30% 20%
5.Bd3	1	Mahdi Khaled - Robatsch Karl (1989)