der Opa
grandpa
A colloquial and familiar term for the grandfather, that is, the father of one's own father or mother.
Jeden Sonntag gehen wir meinen Opa besuchen.
Every Sunday we go to visit my grandpa.

grandpa
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grandpa
A colloquial and familiar term for the grandfather, that is, the father of one's own father or mother.
Jeden Sonntag gehen wir meinen Opa besuchen.
Every Sunday we go to visit my grandpa.

'Opa' (grandpa) is a simplified, childlike form of 'Großpapa' (grandfather). Similar to 'Papa' (dad/father) or 'Mama' (mom/mother), it belongs to familiar family language.
Als Kind habe ich die Ferien immer bei Opa und Oma verbracht.
As a child, I always spent the holidays with Grandpa and Grandma.
'Opa' (grandpa) is a colloquial and emotional word. In formal situations or in an official text, you would use 'Großvater' (grandfather) instead.
Mein Opa erzählt immer die besten Geschichten von früher.
My grandpa always tells the best stories from the old days.
You can often tell from the article whether someone's own grandfather or a strange old man is meant. Possessive pronouns like 'mein Opa' (my grandpa) or 'ihr Opa' (her/their grandpa) usually refer to the relative. 'Der Opa' (the old man/grandpa) or 'ein Opa' (an old man/a grandpa) often refers to an unknown older man.
Mein Opa hat mir das Fahrradfahren beigebracht.
My grandpa taught me how to ride a bike.
senior
'Opa' and 'Senior' can both have to do with an older man. However, 'Opa' is mainly a familiar word for one's own grandfather. It can also be used informally for an old man; depending on the tone, that can sound affectionate or somewhat disrespectful. 'Senior' is more neutral and polite, especially when speaking generally about older people, for example in offers, courses, or prices. Learners may confuse the words because both often refer to older men. The important point is: 'Opa' sounds family-like and familiar, while 'Senior' sounds more general and more respectful.

Am Sonntag kommt mein Opa zum Essen zu uns.
On Sunday, my grandpa is coming to our place for dinner.
Im Schwimmbad bekommen Senioren am Vormittag einen günstigeren Eintritt.
At the swimming pool, senior citizens get cheaper admission in the morning.