Trailing twelve months (TTM) is a figure summed or measured over the most recent twelve-month period ending at the latest reporting date, rather than over a calendar or fiscal year.
A trailing figure moves forward every quarter. When a company reports its third quarter, the trailing twelve months are the fourth quarter of the previous year plus the first three of the current one; a quarter later the window drops the oldest quarter and adds the newest. For a flow such as gross services volume or revenue, the trailing total is the sum of the last four quarters. For a count it is a definition: Upwork Inc. defines an active client as one that spent on the marketplace in the trailing twelve months, and GSV per active client as that spend over the same window.
The trailing figure is useful because a single quarter is noisy and a calendar year is stale for most of the year. It is not a forecast: it says what happened in the last four quarters, not what the next four will bring. And because it blends four quarters, it lags a turn in the trend; a year-on-year comparison of the latest quarter shows a change earlier than the trailing total does.
How Upwork Insights uses it
The Pulse page adds the last four reported quarters to give a trailing GSV only when all four are on file, $3.89B at the latest reading, and states Upwork's own trailing-twelve-month counts as published: 717,000 active clients spending $5.4K each. The GSV stat page sets the trailing total against the $3.13B of lifetime earnings shown by the 28,921 agencies in this index.