Formally, alpha is the intercept from regressing your returns on one or more benchmark returns. If your fund made 14% while the market made 12% and your beta-estimation gives 1.1, the market explains 13.2% and your alpha is about 0.8% before costs.
That definition is deflating on purpose. A great deal of what looks like skill is leveraged beta-estimation, a short volatility profile, or a small-cap tilt. Regressing against the obvious factors before claiming alpha is basic hygiene.
Alpha is also not permanent. See alpha-decay: the more capital chases an effect, the smaller the residual becomes, and published effects tend to shrink by roughly half after publication.
Related: alpha-decay