A more thorough explanation:

TERTIA

tertia (tər-shə), n. Hist. A third. A widow's terce was usually referred to as tertia rationabilis (“a reasonable third”).

Tertia means "a third" in history. For example, if there were three siblings, each would receive a tertia of their father's inheritance. In the past, when a man died, his wife would receive a tertia rationabilis, which means "a reasonable third" of his property. This was to ensure that the widow would have enough to live on after her husband's death.

When a farmer died, his three sons would each receive a tertia of his land.

After her husband's death, the widow received a tertia rationabilis of his estate.

These examples illustrate how tertia was used in the past to divide property or assets into three equal parts.