die Flanke
flank
The side part of a human or animal body between the ribs and the hip.
Der Boxer erhielt einen Treffer in die linke Flanke.
The boxer took a hit to the left flank.

flank / cross
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flank
The side part of a human or animal body between the ribs and the hip.
Der Boxer erhielt einen Treffer in die linke Flanke.
The boxer took a hit to the left flank.

The word 'Flanke' (flank) always means a kind of 'side'. The context makes clear which side is meant: the side of a body, a military formation, a mountain, or a playing field.
Der Soldat sicherte die linke Flanke der Einheit.
The soldier secured the unit's left flank.
In soccer, this kind of pass is often combined with the verbs 'schlagen' (to hit/send), 'geben' (to give), or 'bringen' (to deliver). People speak of a 'hohe Flanke' (high cross) or a 'scharfe Flanke' (driven cross).
Der Außenspieler schlug eine perfekte Flanke in den Strafraum.
The winger sent a perfect cross into the penalty area.
In strategic contexts such as the military or team sports, 'Flanke' (flank) is often used with verbs like 'angreifen' (to attack), 'sichern' (to secure), or 'decken' (to cover).
Der General befahl, die gegnerische Flanke anzugreifen.
The general ordered them to attack the enemy flank.
When talking about mountains, 'Flanke' (flank) is often combined with compass directions to describe a specific side. Typical adjectives are 'steil' (steep) or 'felsig' (rocky).
Die Wanderroute führt entlang der sonnigen Südflanke des Berges.
The hiking route runs along the sunny south flank of the mountain.
page / side
'die Seite' and 'die Flanke' can both refer to a side area. 'Seite' is the general word and fits almost anything: the left or right of a person, a room, a street, or an object. 'Flanke' is more specific. It is used for the side of the body between the ribs and the hip, in sports or military contexts for a side position, and also for the slope of a mountain. Learners may confuse the words because both describe something on the side. A simple rule is: for normal everyday situations, you usually use 'Seite'; 'Flanke' fits better in certain technical or sports contexts.

Beim Training spielt der Verteidiger auf der rechten Flanke.
During training, the defender plays on the right flank.
Yara stellt den Stuhl auf die linke Seite des Tisches.
Yara puts the chair on the left side of the table.