It’s a tweed blazer with a muted plaid (check) pattern.

More specifically:

The base is a brown-olive tweed with lots of tonal variation, giving it that speckled, textured look;

Over that is a soft glen check / plaid, formed by darker intersecting lines.

There’s also a very subtle blue overcheck (windowpane)—thin blue lines that add contrast without popping too loudly.

Overall vibe: classic, earthy, and academic—very British countryside / Ivy-adjacent, not flashy, but rich up close.