Is it more likely that Mr. Trump was earnestly concerned about protecting American interests by pressing for the investigations? Or was he instead looking for personal gain and, in the case of the Bidens, cynically seeking to engineer a basis to be able to call his political rival “corrupt” on the 2020 campaign trail?

If the latter, one way of thinking about the matter is that Mr. Trump solicited a bribe: He was seeking a personal favor from Mr. Zelensky that would benefit him as a condition for performing two official acts — scheduling a White House visit that Mr. Zelensky coveted and releasing military aid that Congress had appropriated.

Why does it matter if this is “bribery” specifically?

Primarily for political messaging. Critics of Mr. Trump have generally been talking about the scandal in terms of more abstract concepts like “abuse of power” and the Latin phrase “quid pro quo,” which means exchanging one thing for another.

Those phrases can be difficult to understand and raise the question of whether they amount to an impeachable offense. Ms. Pelosi’s sharpened rhetoric was part of a shift in which Democrats and other critics of Mr. Trump have sought to talk about their allegations using a more plain-English term — and one that is explicitly impeachable.

What about extortion?

Some analysts have cited extortion as another framework for thinking about Mr. Trump’s pressure campaign on Ukraine. One version of this offense is when a public official unlawfully obtains something of value, like an illegal fee, from another person by threatening to take or withhold some official action in a way that would harm the victim.

Last month, when Mick Mulvaney, Mr. Trump’s acting chief of staff, gave a news conference in which he appeared to admit that the arrangement was a “quid pro quo” — which he later walked back — he also told reporters to “get over it” because it is routine for the United States government to hold up foreign aid to get a recipient country to change some policy.

As a description of using foreign aid as a lever in foreign policy, Mr. Mulvaney was correct. But the question boils down to whether Mr. Trump was seeking to coerce Ukrainian officials into actions to benefit the United States or to benefit himself.