"will, determination, esp. of the gods," "counsel, design," "generally, counsel, advice," and "deliberation" or "decree." But it's clear that context must determine which definition fits best. BDAG does not seem to advocate "plan" either although that lexicon demonstrates that "plan" is within the term's lexical range. However, I would submit that the meaning does not fit the verses in Acts.
The New Linguistic and Exegetical Key to the Greek New Testament (Rogers and Rogers) states that βουλὴ (Acts 2:23) evidently means (has the sense): "purpose, decision, counsel."
The Expositor's Greek Testament:
βουλῇ: Wendt compares the Homeric διὸς δʼ ἐτελείετο βουλή. The phrase βουλή τοῦ θ. is used only by St. Luke; once in his Gospel, Acts 7:30, and three times in Acts 13:36; Acts 20:27 (whilst βουλή is used twice in the Gospel, eight times in the Acts, and only three times elsewhere in the N.T., 1 Corinthians 4:5, Ephesians 1:2, Hebrews 6:17), but cf. Wisdom of Solomon 6:4; Wisdom of Solomon 9:13, and often ἡ βουλή κυρίου in LXX.
We could add Joseph Alexander's commentary on Acts where he writes: "The word translated counsel properly means will, as appears both from etymology and usage" (p. 70). compare Job 38:2; 42:3 Isa 5:19; 9:6; 46:10 (LXX).
Finally, Robertson's Word Pictures on Acts 2:23:
"By the determinate counsel and foreknowledge of God (τηι ωρισμενηι βουληι και προγνωσηι του τεου — tēi hōrismenēi boulēi kai prognōsēi tou theou). Instrumental case. Note both purpose (βουλη — boulē) and foreknowledge (προγνωσις — prognōsis) of God and 'determined' (ωρισμενη — hōrismenē perfect passive participle, state of completion). God had willed the death of Jesus (John 3:16) and the death of Judas (Acts 1:16), but that fact did not absolve Judas from his responsibility and guilt (Luke 22:22). He acted as a free moral agent."