das Kreuz
cross
A geometric figure or sign consisting of two intersecting lines, usually at a right angle.
Bitte machen Sie an der gewünschten Stelle ein Kreuz.
Please put a cross in the desired place.

cross
Loading your saved state.
cross
A geometric figure or sign consisting of two intersecting lines, usually at a right angle.
Bitte machen Sie an der gewünschten Stelle ein Kreuz.
Please put a cross in the desired place.

'Das Kreuz' (cross) is related to the verb 'kreuzen' (to cross) and the noun 'die Kreuzung' (intersection). All of these words are based on the idea of intersecting lines.
Unsere Wege haben sich nach der Schule leider nie wieder gekreuzt.
Unfortunately, our paths never crossed again after school.
The many meanings of 'Kreuz' (cross) can be recognized well from context. If it is about religion, the symbol is meant. With pain, it means the back. On a form, it is a mark to tick or check.
Er hat seit Wochen Schmerzen im Kreuz.
He has had pain in his lower back for weeks.
The expression 'drei Kreuze machen' means that you are very happy and relieved when an unpleasant situation is finally over.
Als die Prüfung endlich vorbei war, habe ich drei Kreuze gemacht.
When the exam was finally over, I was incredibly relieved.
In the meaning 'Zeichen' (mark/sign), the verb 'machen' (to make) is almost always used. You 'macht ein Kreuz' on a ballot, in a form, or on a test.
Machen Sie bitte bei der richtigen Antwort ein Kreuz.
Please put a cross next to the correct answer.
When 'Kreuz' means the back, it is often connected with pain. Typical expressions are 'Probleme mit dem Kreuz haben' (to have back problems) or, colloquially, 'es im Kreuz haben' (to have lower back pain).
Seit ich im Garten gearbeitet habe, habe ich es im Kreuz.
Since I worked in the garden, I've had lower back pain.
In the expression 'sein Kreuz tragen' (to bear one's cross), the possessive pronoun is adapted to the person. So it is 'ich trage mein Kreuz' (I bear my cross), 'du trägst dein Kreuz' (you bear your cross), and so on.
Jeder Mensch muss im Leben sein eigenes Kreuz tragen.
Every person must bear their own cross in life.
back
'der Rücken' and 'das Kreuz' can both refer to the back part of the body, especially when someone is in pain. 'der Rücken' is more general and refers to the area from the neck to the buttocks. 'das Kreuz' more specifically means the lower back. That is why people often say 'Ich habe Rückenschmerzen' (I have back pain) when they mean the whole area. 'Mir tut das Kreuz weh' (my lower back hurts) fits better when the pain is in the lower back. The words can be confused because both occur in everyday situations involving pain and strain.

Nico hebt die schwere Kiste hoch, obwohl ihm das Kreuz wehtut.
Nico lifts the heavy box even though his lower back hurts.
Nach dem langen Sitzen tut mir der ganze Rücken weh.
After sitting for so long, my whole back hurts.