A buying signal is an observable event at an account that suggests it has entered a buying window. A funding round, a hiring surge, a new tool in the stack, a leadership change. The event is public and datable, which is what separates a signal from a guess about who might be interested.
Why it matters
Signals answer a different question from firmographics. Firmographics tell you an account looks like your customers. A signal tells you something changed there recently, which is the only part that argues for contacting them this week rather than next quarter.
That difference is also the whole justification for the first line of a cold email. "You raised a Series A eleven days ago and posted three sales roles" is a reason. "You are a 50-person SaaS company" is a filter that half your market shares.
Example
A company posts four sales roles in a fortnight, having posted none for six months. That is a hiring signal. It predicts, fairly reliably, that someone has approved sales headcount and a budget cycle has opened. It does not predict that they want your product, that the budget is discretionary, or that anyone there has heard of you.
Common mistake
Treating every signal as equally strong, and treating "strong" as one property. A signal has at least three independent dimensions: how recent it is, how well the account fits your ICP, and how much the event actually implies about budget. An account can score high on all three or high on one, and a scoring model that collapses them into a single number will happily rank a perfect-fit account with a nine-month-old event above a fresh event at a company you cannot sell to.
How this relates to Signl
Signl tracks four company-level signal types: funding, hiring, tech_adoption and company_match. It also derives four person-level types on contacts you have already enriched: job_change, promotion, tenure_milestone and person_content.
Every signal carries both when the event happened and when Signl first detected it, so you can see the gap rather than trusting a single freshness number.