das Glück
happiness
A state of intense joy and contentment.
Für sie bedeutet das größte Glück, Zeit mit ihren Enkelkindern zu verbringen.
For her, the greatest happiness means spending time with her grandchildren.

happiness / luck
Loading your saved state.
happiness
A state of intense joy and contentment.
Für sie bedeutet das größte Glück, Zeit mit ihren Enkelkindern zu verbringen.
For her, the greatest happiness means spending time with her grandchildren.

'Glück' has two central meanings: an inner feeling of joy and contentment (happiness) or a positive coincidence (luck). The context makes clear which meaning is intended.
Er fand sein Glück in der Musik.
He found his happiness in music.
To express that someone experiences a fortunate coincidence, you use the fixed expression 'Glück haben' (to be lucky). You cannot say 'Glück sein'.
Wow, du hast aber Glück gehabt!
Wow, you really were lucky!
The expression 'zum Glück' functions like an adverb and means 'fortunately'. If it stands at the beginning of a sentence, the verb follows directly after it in position 2.
Ich dachte, ich hätte meinen Schlüssel verloren. Zum Glück war er in meiner anderen Jacke.
I thought I had lost my key. Fortunately, it was in my other jacket.
The wish 'Viel Glück!' almost always means 'Good luck!', that is, the wish for a positive coincidence. It is used before exams or challenges, not to wish someone general joy in life.
Du hast morgen dein Vorstellungsgespräch? Viel Glück!
You have your job interview tomorrow? Good luck!
unfortunately
'leider' and 'zum Glück' express opposite feelings about a situation. You use 'leider' when you regret something or are disappointed. You use 'zum Glück' when you are relieved because something good happened or something bad did not happen. So both words comment on the whole sentence. The difference is the evaluation: 'leider' is negative, while 'zum Glück' is positive or expresses relief.

Zum Glück hat die Familie den Schlüssel wiedergefunden.
Fortunately, the family found the key again.
Leider fällt der Deutschkurs am Montag aus.
Unfortunately, the German course is canceled on Monday.