The Lions use the term WILL to refer to the linebacker on the short or weak side of the field, where most running plays go. This has been Anton McKenzie's position all year in the 4-2 defence and the 4-3.
The Lions use MAC for middle linebacker, who lines up inside the defensive end on the wide side of the field in 4-2 defence or near the middle of the formation in a 4-3. This was Javy Glatt's position (most often in a 4-2 defence) until JoJuan Armour took over three games ago.
The Lions occasionally use a SAM or strong-side outide linebacker in a 4-3 defence. This is mainly where Javy Glatt has played in recent games. In a 4-2 defence, Korey Banks plays a version of that role, but is called the nickelback. He may line up on the strong side and occasionally blitzes from there, but he often has man coverage responsibilities on the running back or an inside receiver.
Here's B.C.'s depth chart for the Montreal game showing the abbreviations for the positions. Note that the "weak" designation is also used for defensive end (Brent Johnson) who plays on the short-side of the field and the cornerback (Dante Marsh) who plays that side.Link
"I think [Fred Fateri] was like a lot of people who watch football, hockey or any professional sport on television. They sit there and think they could coach better than the professional. Some people really think that." - Bob Ackles, The Water Boy