一个
yī ge
a; an; one
the whole (afternoon, summer vacation etc)
one (一) + counter (个) = a single item
The character 个 is a measure word that pairs with nouns, making 一个 the standard way to say 'one' of something countable. Think of 个 as a universal counter that works with most everyday objects.
我有一个苹果。
wǒ yǒu yī ge píng guǒ.
I have an apple.
他花了一个下午来做作业。
tā huā le yī ge xià wǔ lái zuò zuò yè.
He spent the whole afternoon doing homework.