They are officially categorised as mechanised infantry, and the NATO standard icon for them is the Mech Infantry icon of an oval within a square with an 'X'. (With wheels on the oval) Motorised infantry basically means unarmoured trucks. (Recently resurrected by the US Army when it redesignated TWATs and other such troops who had their normal vehicles taken away and turned into infantry in HMMWVs as "Motor Infantry"), re-redesignated as Dragoons. (NATO Icon: "X" in a square, with wheels). FWIW, IFVs such as Bradley have an Oval and X in a square with a vertical line on the left signifying their autocannon, and tracked APCs such as M113 are simply an oval and X in a square.
An old, old term might be 'Armoured Infantry', but that designation didn't last much after WWII.
Strykers are indeed considered medium-weight APCs, although the emergence of a new category of 'Heavy APC' in recent years has led some to consider redefining vehicles such as Bradley and Marder, (technically IFVs), as mediumweight although they are more armoured and heavier than vehicles such as Stryker.
NTM