A subdivision is any legal division of one parcel into two or more parcels, recorded so each new parcel can be described, sold, taxed, and insured on its own. A plat is the surveyed map showing the new lot lines, and recording that map with the county is what makes the new lots real in the eyes of the recorder and a title insurer.
Most counties sort divisions into two tracks. The minor track covers a small number of lots where no new public road is being created, and planning staff can sign off without a hearing. The major track covers everything else, and that's where you get engineering plans, hearings, and conditions about roads, drainage, or utilities. "Lot split" is ordinary market shorthand for the minor track. Your title guy wasn't wrong about the practical difference, he was using a word your ordinance doesn't. Thresholds and names are set locally and differ by state as well as county, so the only version that governs your deal is the one in your county's text.
The part that catches people is that meeting the zoning minimum lot size isn't the same as being able to divide. Each new lot usually needs legal access to a public road, its own road frontage measured a specific way, and in most rural counties a soil test showing a septic system will work. A 20 acre parcel in a 5 acre minimum zone can look like four lots on paper and be two in practice because only 300 feet of it touches the road. Read the access and frontage sections before you read anything else.
