What is “en passant” (french for ‘In Passing’)?

As is very well illus­trat­ed / ani­mat­ed at Wikipedia, It is a spe­cial pawn cap­ture that can only hap­pen if the con­di­tions are cor­rect. It is pro­nounced: en pas·​sant.  The his­to­ry of en pas­sant with exam­ples before 15th cen­tu­ry & after 15th cen­tu­ry, plus how some­thing unfair became fair as it is today. Here’s anoth­er good exam­ple of en pas­sant. Also ‘Accord­ing To Hoyle’ ( [this can be viewed in the book on page 237] — … is a very much a strate­gic move which forces the oth­er play­er to make up his or her mind on what to do.):

‘EN PASSANT’ or ‘in pass­ing’ can only be done on the very move fol­low­ing the move that the pawn made by try­ing to pass the oth­er pawn. In the exam­ple below, the black pawn ‘is not per­mit­ted escape this attack by use of the dou­ble move. — Had it  moved only one square, it would been attacked’. This works for both sides of a pawn.
Rules for en pas­sant cap­ture are:

1. If a pawn makes a dou­ble-jump, it can be cap­tured by an adverse pawn that could have cap­tured it had it advanced only one square.
2. The en pas­sant cap­ture must be exe­cut­ed imme­di­ate­ly or not at all; it may not be made at any lat­er turn.

Below is the example for both 2D and 3D (top board = ‘A’, bottom board = ‘B’):

A Dimensional en-passant

White Pawn: Bh2 — Bh4

Black Knight try’s to cap­ture Bh4: Ag8 — Ah6

White Pawn avoids cap­ture by mov­ing to 5th rank: Bh4 — Bh5

Black­’s Pawn try’s to Pass white’s pawn on H5: . Ag7 — Ag5.  Because If Ag7-Ag6 then Bh5 x Ag6

Only at this moment: White Pawn: Bh5 x Ag5 . .  (a Dimen­sion­al En-Passant).

Going back one move, what if Black try’s to pass on the bot­tom board lev­el, ‘B’?
This is, as if it were a reg­u­lar 2D flat game board.

Black­’s Pawn try’s to Pass on the bot­tom board: Ag7 — Bg5.

Only at this moment:  White Pawn:  Bh5 x Bg5  (En Passant)

Here is a good visu­al on YouTube for En Pas­sant.