In everyday parlance, you are free to use the word “estate” loosely. Wineries often use the word "estate" or "wine estate" in their names, and the word has a variety of contexts and meanings inside and outside the world of wine.
On a wine label, however, the word "estate" has a very specific and legally-regulated meaning. Estate wine, from a regulatory, compliance, and labelling perspective, is wine which was grown, made, and bottled on a single contiguous property. This excludes wines made from various parcels in different geographies and wine made from grapes grown by another company or person. To say "estate" anywhere on a wine label, you have to grow the grapes yourself, and the wine has to come from a single origin.
Recently some South African wineries have lobbied to change this regulation to allow them to use the word "estate" as part of their trading name on wine labels. So, one could theoretically see a wine produced by "Acme Wine Estate" which was not made from grapes grown there.
In my opinion, this was a misguided and nonsensical proposal. Fortunately, this proposal was defeated through some lobbying by family-owned and estate wineries who value the meaning of the term "estate".
Protected geographical indications are the building blocks of terroir, and the reason why you can have confidence that a wine that says "Constantia" on the label really does come from that region. Estates (and single vineyards) play a major role in defining the characteristics and parameters of regional units.
Estate wine also gives the consumer confidence in viticultural consistency from vintage to vintage. Estate wine cannot be blended and made to fit a profile to the same degree as wine from various parcels, and this gives each estate wine its unique thumbprint and identity.
Estate wine is something worth valuing, and something worth protecting. When you see "Estate" on a wine label, you should have confidence that the term carries a specific and definable meaning.