Rashi on Leviticus 23:10/רש"י על ויקרא כ״ג:י׳

1ראשית קצירכם THE FIRST FRUITS OF YOUR HARVEST — This means that it (the Omer) shall be the first thing to be harvested (cf. Sifra, Emor, Section 10 3).

2עמר — The tenth part of an Epha; this was its name (i. e. עמר has not the meaning of a sheaf here as is the meaning of the word e. g., in Deuteronomy 24:19: ושכחת עמר — but it denotes a certain measure), as we find (Exodus 12:18) “And they did measure it with the Omer”.