How would you classify this blunder?

Please refer to Game 1612392. I was able to hang on for a draw, but I nevertheless blundered with 26...b5. I did a mini-analysis after the game, and I post it for your review as well:

10... Qa5 was a silly threat, but I felt he weakened by avoiding the more traditional retreat of Nb3. This would have offered Nxe4 with a discovered attack on his pinned C3 knight.

14... Na5 is a little dubious, but I wanted to own c4. Why c4? I had three attacks, and he has only one defender. I wanted an eventual Nc4 with a fantastic queenside post if he advanced his b pawn.

14. Ng5. I did see that g5 was weak, but I saw no move that would improve it. I expected him to take his post with a looming threat.

15... Bc4. Simply attempting to capitalize on my ownership of c4. I decided to attack with my bishop for a few reasons: One, it pins his bishop to his rook, so he won't retreat; two, I reasoned that an eventual Nxc4 would achieve a nice post for my knight as I had wanted. of course the anticipated white move b3 would compromise that post, so I'd have to deal with that.

24...Ne5. I thought this was a rather dubious move on my part.

26...d5?? A tactical blunder. I made this move with haste, but I made sure that it was properly defended; it was, but I didn't anticipate 27. Qd2 introducing another attacker, and my pinned knight has nowhere to go.

I was frustrated after 26...d5, so I decided to unleash a pawn storm on his for a few reasons: One, I have the pawn majority; two, I have substantially more space with my pawns; three, I wanted to take advantage of his likely psychological state of a won-game.

Any other thoughts? How would you classify my blunder anyway? Any key errors that I missed? Thanks!

You threw away a game already won.

At the end, if white takes the pawn then the point count is exactly
the same, 12. Thought black gets a pawn but was wrong.
so it would be a fight but black may still fork one of the white pieces.
26...d5 looks like was done because of the threatened rook fork
but of course almost anything would have been better, like
26...Q-e7 or R-e8 to protect the forking pawn at e6.

Originally posted by Tetsujin
You threw away a game already won.

Yo played 26. ... d5, not b5, that was on move 25. 26.d5 was a blunder indeed, going into a pinning which cost you apiece. What type of blunder? Carelesness perhaps?

Originally posted by sonhouse
At the end, if white takes the pawn then the point count is exactly
the same, 12. But black gets the rook pawn after that and may
get both remaining pawns and it would be a fight after that,
black could even win, forking one of the pieces with the power of
the queen. Black gets the rook pawn by checking at c1.

I would say that if you did the following you would have won:
51. Rg3 Rg4xg3+
51. Kxg3 g1=Q+
This way you promote your pawn whilst avoiding perpetual check.

Originally posted by Mephisto2
Yo played 26. ... d5, not b5, that was on move 25. 26.d5 was a blunder indeed, going into a pinning which cost you apiece. What type of blunder? Carelesness perhaps?

Originally posted by Chesswick
My fault. I fixed it. Thanks!

I agree that it was carelessness, but only if you consider potential replies. The move was safe considering the current position, but I failed to acknowledge his adding another attacker to my knight.

I guess I'm just looking to classify the blunder so that I can more easily prevent it. It wasn't a counting error ...[text shortened]... ddition to all the more obvious threats of checks, tactical possibilities, pieces en prise, etc.

Originally posted by sonhouse
You were trying to prevent the bishop fork so hid it with d5 but
Q-e7 or R-e8 does the same thing.
The end may have still been a win for black, at move 52:
instead of 52...h6 go 52...K-f6, no more checks
then if 53 Rxg2? Q-f5+ wins bishop and perhaps the game.