'Twas is a contraction of “it was,” used mostly in poems and old-fashioned stories. You've probably heard it in the famous line: 'Twas the night before Christmas, when all through the house...

Writers use 'twas partly because it sounds poetic and old-timey, and partly because it's shorter than “it was,” which helps when you're counting syllables in poetry. If a poet needs their line to have exactly ten syllables, 'twas saves them one precious beat.

You'll rarely hear anyone say 'twas in normal conversation today (unless they're being deliberately fancy or funny). But it appears in classic poems, Christmas stories, and historical novels set in earlier centuries. When authors write “'twas a dark and stormy night” instead of “it was a dark and stormy night,” they're giving their writing an antique flavor, like using an old brass key instead of a plastic one.

The apostrophe shows where the “i” got dropped when “it was” got squeezed together. English has lots of these contractions: don't, can't, won't. 'Twas is just one that fell out of everyday use but stuck around in poetry and holiday traditions.