1The money paid for a thing or service. Most often used as a suffix 〜代 meaning 'the charge/fee for ~', as in ガス代 'gas bill', タクシー代 'taxi fare'.
今月は電気代が高かった。
The electricity bill was high this month.
飲み代はみんなで割り勘にしよう。
Let's split the bar tab among everyone.
部屋代を毎月25日に払っている。
I pay the rent on the 25th of every month.
2A generation, age bracket, or cohort. Used to divide age into decades (20代 'one's twenties', 30代 'one's thirties') and to refer to one's own generation or class year (私たちの代).
父は60代になっても元気だ。
My father is still healthy even in his sixties.
この商品は20代の女性に人気がある。
This product is popular with women in their twenties.
私たちの代から制服が変わった。
The uniform changed starting with our year.
3In geology, a major division of Earth's history — an era, such as the Paleozoic (古生代), Mesozoic (中生代), or Cenozoic (新生代).
恐竜が栄えたのは中生代だ。
It was in the Mesozoic era that dinosaurs flourished.
人類が現れたのは新生代になってからだ。
Humans appeared only in the Cenozoic era.
4Placed after a name or title to mean 'acting on behalf of / as a proxy for'; often written as （代） in documents and minutes.
社長が欠席のため、書類には「社長（代）」と記入した。
As the president was absent, the document was filled in as 「President (proxy)」.
5A notation placed after a phone number to show it is the company's main (representative) line, written as （代） on business cards and the like.
名刺には会社の代表番号として「03-1234-5678（代）」とある。
The business card lists the company's main line as 「03-1234-5678 (代)」.
6A counter for grouping ages into decades, and (in 〜年代) for decades of an era — 40代 'one's forties', 1990年代 'the 1990s'.
彼は40代後半に入った。
He's entered his late forties.
1990年代に流行した曲をよく聴く。
I often listen to songs that were popular in the 1990s.
7A counter for successive generations (of a family, line, throne, etc.) — 三代続く 'to continue for three generations', 代々 'generation after generation'.
この旅館は三代続いている。
This inn has been run for three generations.
徳川家は十五代まで続いた。
The Tokugawa house lasted for fifteen generations.
8An abbreviation in which 代 stands for 代理申請会社 ('proxy application company'), noted on administrative documents. A specialized, limited usage.
9In dictionary classifications, 代 is sometimes used as an abbreviation for 代名詞 ('pronoun'). It is a part-of-speech label, not a word used in running text.