ERP is the unhelpful name for a simple wish: one set of facts for the work of the business. An order, the stock it needs, the job it creates, and the invoice it becomes should be the same story, not four spreadsheets that almost agree at the end of the month. The letters stand for enterprise resource planning. Nobody in the office needs the expansion. They need to stop reconciling.
One story, several desks
Sales promises a date. Stores knows what arrived. Production knows what was made. Accounts knows what was billed. If each of those lives in its own workbook, the company does not have a system. It has four memories and a meeting. An ERP, whether you buy it or build it, is the place those memories are forced to use the same customer, the same item, and the same order number.
Tally, or whatever holds the books, often stays the ledger. The ERP does not have to replace the statutory accounts on day one. It has to stop people typing the same sale into the ledger by hand because nothing else will. That boundary should be written down. Otherwise the project becomes "replace everything" and never goes live.
A product or a system of your own
The word does not tell you which. A package is an ERP if it covers the jobs and you are willing to work its way. A custom system is an ERP if it covers the jobs the way you already win the work. The expensive mistake is a package you then customise until it is a private fork you do not own, or a custom build of something a product already did well.
If the argument in the room is about logos, read custom ERP versus SAP versus Odoo before you take three demos that are not about the same problem.
When it matters on a project
It matters when the afternoon is spent explaining why two numbers differ. It does not matter when the pain is one report and the books are fine. Start with the workflow people actually fight over, not with a module list. A first slice that sales and stores both trust is an ERP. A login nobody opens is a brochure.
We build that kind of system under ERP when the process is the point. If a standard product fits and the team will change the form, say so early and buy it. The glossary entry is the definition. The project is the first number two departments will both sign.