Catherine Parr; a "commoner" in Tudor Times?

What exactly did it mean back in the Tudors reign to be a "commoner"? Has it changed at all over the years -- because someone I was talking with insists that it just means 'not royal' -- which is not correct. From what I know a commoner is anyone who does not hold a title in the peerage or is not the monarch. If Katherine Parr had been married and became Lady Latimer she was no longer considered a commoner

Normally one refers to or addresses Baron [X] as Lord [X] and his wife as Lady [X].

-- and after his death became Dowager Lady Latimer. In her biographies, it is quoted that she is the first woman besides her great-aunt, Mabel Parr who became Lady Dacre, to marry into the peerage and receive a title.

From William Charteris Macpherson, ''The Baronage and the Senate: or The House of Lords in the Past, the Present, and the Future'' (London: John Murray, 1893):

In British law, a commoner is someone who is neither the Sovereign nor a peer.