PM here means Prime Minister: the head of the government in countries such as the United Kingdom, Canada, Australia, India, and Japan. It is a short way to write or say the title, especially in headlines and spoken news — 'the PM said…'. As a label it feels a little informal; in a careful letter you would write Prime Minister in full, but everyone knows PM. In British talk, 'the PM' often means the UK leader, as if there were only one that mattered that day. Different countries give this job different Chinese titles, but the English letters stay PM. This entry is only the government leader. The same two letters also mean the afternoon (p.m.) and, online, a private message — those are other uses, filed elsewhere.