A trillion is a large number that has two common definitions depending on the numbering system used.

• Short scale (most of the world, including the United States): a trillion equals 1,000,000,000,000 (one followed by 12 zeros), or 10^12. It is also described as one thousand billion.
• Long scale (used historically in parts of Europe and in some contexts today): a trillion equals 1,000,000,000,000,000,000 (one followed by 18 zeros), or 10^18.

Key points:

• In everyday financial and scientific contexts in the US and many other countries today, "trillion" typically means 10^12.
• Some older or non-US usages may refer to 10^18, especially when contrasted with a billion (which itself can be 10^9 on the short scale or 10^12 on the long scale).

Common ways people think about it:

• 1 trillion dollars = 1,000,000,000,000 dollars.
• A million million = 1 trillion in the short scale.

If you’d like, I can tailor this with examples (e.g., comparing trillions to billions and to other large numbers) or provide a quick conversion chart.