"Tent stitch" and "cross stitch" get used almost interchangeably in casual conversation, and a single diagonal tent stitch does look identical to a half cross stitch on the front of the fabric. But they come from different traditions, use different base fabric, and aren't really the same technique.
The core difference
Cross stitch is worked on evenweave fabric — Aida, linen, or evenweave cloth with an open, countable weave — using floss, and is built from a pair of crossing diagonal stitches (or a single diagonal for half cross stitch). The fabric threads themselves are the counting grid.
Tent stitch is a needlepoint technique, worked on canvas (a stiff, open-mesh fabric meant to be fully covered) using wool or a heavier thread, and is a single diagonal stitch — never crossed — worked in one of several specific patterns (continental, basketweave, or half-cross tent stitch) chosen for how they affect canvas distortion and thread coverage on the back.
Why the confusion happens
Needlepoint's "half-cross tent stitch" variant is worked identically to a cross-stitch half stitch — same diagonal, same direction — which is exactly why the two get conflated. The difference that matters practically is the fabric: canvas needlepoint is meant to be entirely covered edge to edge (it's a painting surface, essentially), while cross-stitch charts deliberately leave fabric bare as part of the design. Using tent stitch technique on canvas with cross-stitch floss (or vice versa) will technically produce stitches, but the tension, coverage, and finished feel will be off for the intended result.
The practical takeaway
If a kit or pattern says "needlepoint" or specifies canvas and wool/tapestry yarn, it's a tent stitch project. If it specifies Aida, linen, or evenweave and embroidery floss, it's cross stitch. Everything else on this site — techniques, materials, and guides — is written for the cross-stitch tradition specifically.
What it's used for
Knowing which stitch a pattern or kit actually calls for — the two are visually similar enough (and confused often enough in casual use) that it's worth knowing the difference before buying materials.