1Being equal or identical in quantity, nature, or condition -- a somewhat formal word often used in mathematical or logical contexts.
二つの箱の重さは等しい。
The two boxes weigh the same.
彼らの給料は等しく支払われるべきだと主張した。
He argued that they should be paid equally.
この二つの数式は等しい。
These two formulas are equal.
2Used in the form '~と等しい' to mean there is essentially no difference from something else -- 'the same as' or 'tantamount to'.
彼の無断欠勤は事実上の退職と等しい。
His unauthorized absence is, in effect, tantamount to resignation.
この結果は失敗と等しい評価を受けた。
This result was rated as tantamount to failure.