1A picnic — an outdoor leisure activity that involves bringing food to eat while relaxing in a park or other outdoor spot.
週末に家族とピクニックに行ってお弁当を食べた。
We went on a picnic with my family on the weekend and ate boxed lunches.
桜の季節には公園でピクニックをする人が多い。
Many people have picnics in the park during cherry blossom season.
ピクニック用のレジャーシートを忘れずに持ってきてください。
Don't forget to bring a picnic sheet.
2A dated sense meaning an outing or hike. Rarely used in modern Japanese, where "ピクニック" now almost always refers to an outdoor meal (see sense 1).